Many of us use PDFs every day. They’re great documents to work with and read from because of their ease of use and ease to create.
I think I started to use PDFs for the first time in 2004. That’s when I got my first computer. Since that time, most PDFs I have needed to use have just worked. In the time that I have been using PDFs I have noticed that there are at least two major ways in which PDFs are not created equally:
- Validity of the PDF: Adherence to the PDF document standard.
- Resolution of contained images
- The presence and accuracy of the PDF’s meta-data.
Since 2004, there have only been a few PDFs which after creation and distribution would not render by any of my PDF readers, or on the readers my friends used (most of these PDFs were created by Microsoft Word or Microsoft Publisher on Windows and actually one or two created by Apple’s word processor Pages). Sometimes these errors had to do with a particular image included in the source document. The image may have been malformed, but this was not always the case. Sometimes it was the PDF creator, which was creating non-cross-platform PDFs.
Not all PDFs are created equal. (This is inherently true when one considers the PDF/A The University of Michigan put a small flyer together on how to get something like a PDF/A to print from MS Word on OS X and Windows. [Link], and PDF/X standards, however lets side-step those standards for a moment.) To frame this discussion, it is necessary to acknowledge that there is a difference between creating a digital document with a life expectancy of 3 weeks and one with a life expectancy of 150 years. So for some applications, what I am about to say is a moot point. However, looking towards the long term…
If an archival institution wants a document as a PDF, what are the requirements that that PDF needs to have?
What if the source document is not in Unicode? Is the font used in the source document automatically embedded in the PDF upon PDF creation? Consider this from PDFzone.com
Embedding Fonts in a PDF
Another common area of complaint among frequent PDF users is font incompatibility and problems with font embedding. Here are some solutions and tips for putting your best font forward so to speak.
Keep in mind that when it comes to embedding fonts in a PDF file you have to make certain that you have the correct fonts on the system you’re using to make the conversion. Typically you should embed and subset fonts, although there are always exceptions.
If you just need a simple solution that will handle the heavy font work for you, the WonderSoft Virtual PDF Printer helps you choose and embed your fonts into any PDF. The program supports True Type and Unicode fonts.
The left viewing window shows you all the fonts installed on your system and the right viewing window shows the selected user fonts to embed into a newly created PDF form. A single license is $89.95.
Another common solution is the 3-Heights Optimization PDF Optimization Tool [Link Removed].
One of the best sources of information on all things font is at the Adobe site itself under the Developer Resources section.
- Embedded PDF executable hack goes live in Zeus malware attacks
- Hacker finds a way to exploit PDF files, without a vulnerability
- Escape From PDF
- Adobe suggests workaround for PDF embedded executable hack
- CVE-2010-1240 : Adobe PDF Embedded EXE Social Engineering
Printing the PDF does not seem to be a fail proof method to see if the PDF is valid or even usable. See this write up from The University of Sydney School of Mathematics and Statistics:
I can read the file OK on screen, but it doesn’t print properly. Some characters are missing (often minus signs) and sometimes incorrect characters appear. What can I do?
When the Acrobat printing dialog box opens, put a tick in the box alongside “print as image”. This will make the printing a lot slower, but should solve the problem. (The “missing minus signs” problem seemed to occur for certain – by now rather old – HP printers.)
(Most of these problems with pdf files are caused by subtle errors with the fonts the pdf file uses. Unfortunately, there are only a limited number of fonts that supply the characters needed for files that involve a lot of mathematics.)
Printing a PDF is not necessarily a fail proof way to see if a PDF is usable. Even if the PDF is usable, printing the PDF does not ensure that it is a valid PDF either. When considering printing as a fail proof method one should also consider that PDFs can contain video, audio, and flash content. So how is one to print this material? Or in an archival context determine that the PDF is truly usable? A valid PDF will render and archive correctly because it conforms to the PDF standard (what ever version of that standard is declared in the PDF). Having a PDF conform to a given PDF standard puts the onus on the creator of the PDF viewer (software) to implement the PDF standard correctly. Thus making the PDF usable (as intended by the PDF’s author).
Note: I have not checked the Digital Curation Center for any recommendations on PDFs and ensuring their validity on acceptance to an archive.
Resolution of Contained Images
A second way that PDF documents can vary is that the resolutions of the images contained in them can vary considerably. The images inside of a PDF can be a variety of image formats, .jpg, .tiff, .png, etc. So the types of compression and the looseness of these compressions can make a difference in the archival “quality” of a PDF. A similar difference is is noted to be the difference in a raster PDF and a Vector PDF. Beside these two types of differences there are various PDF printers, which print materials to PDF in various formats. This manual discusses setting Adobe Acrobat Pro’s included PDF printer.
A third way in which PDFs are not created equally is that they do not all contain valid, and accurate meta-data, it the meta-data containers available in the PDF standard. PDF generators do not all respectfully add meta-data to right places in a PDF file, and those which do sometimes add meta-data to a PDF file do not always add the correct meta-data to the PDF.
Prepressure.com presents some clear discussion on the embedded properties of meta-data in PDFsPreppressure.com has a really helpful section on PDFs and various issues pertaining to PDFs and their use. http://www.prepressure.com/pdf/basics.
Their discussion on meta-data can be found at http://www.prepressure.com/pdf/basics/metadata.
How metadata are stored in PDF files
There are several mechanisms available within PDF files to add metadata:
- The Info Dictionary has been included in PDF since version 1.0. It contains a set of document info entries, simple pairs of data that consist of a key and a matching value. Some of these are predefined, such as Title, Author, Subject, Keywords, Created (the creation date), Modified (the latest modification date) and Application (the originating application or library). Applications can add their own sets of data to the info dictionary.
- XMP (Extensible Metadata Platform) is an Adobe technology for embedding metadata into files. It can be used with a wide variety of data files. With Acrobat 5 and PDF 1.4 (2001) this mechanism was also made available for PDF files. XMP is more powerful than the info dictionary, which is why it is used in a number of PDF-based metadata standards.
- Additional ways of embedding metadata are the PieceInfo Dictionary (used by Illustrator and Photoshop for application specific data when you save a file as a PDF), Object Data (or User Properties) and Measurement Properties.
PDF metadata standards
There are a number of interesting standards for enriching PDF files with metadata. Below is a short summary:
- There are PDF substandards such as PDF/X and PDF/A that require the use of specific metadata. In a PDF/X-1a file, for example, there has to be a metadata field that describes whether the PDF file has been trapped or not.
- The GWG ad ticket provides a standardized way to include advertisement metadata into a PDF file.
- Certified PDF is a proprietary mechanism for embedding metadata about preflighting – whether a PDF file intended to be printed by a commercial printer or newspaper has been properly checked on the presence of all fonts, images with a sufficient resolution,…
The filename is metadata as well
The easiest way to add information about a PDF to the file is by giving it a proper filename. A name like ‘SmartGuide_12_p057-096_v3.pdf’ tells a recipient much more about what the file is about than ‘pages_part2_nextupdate.pdf’ does.
- Add the name of the publication and possibly the edition to the filename.
- Add a revision number (e.g. ‘v3′) if there will be multiple updates of a file.
- If a file contains part of the pages of a publication add at least the initial folio to the filename. That allows people to easily sort files in the right order. Use 2 or 3 digits for the page number (e.g. ‘009′ instead of just ‘9′).
- Do not use characters that are not supported in other operating systems or that have a special meaning in some applications: * < > [ ] = + ” \ / , . : ; ? % # $ | & •.
- Do not use a space as the first or last character of the filename.
- Don’t make the filename too long. Once you go beyond 50 characters or so people may not notice the full information or the filename may get clipped in browser windows or applications.
- Many prepress workflow systems can automatically insert files into a job based on a specific naming convention. This speeds up the processing of the job and can avoid costly mistakes. Consult with your printer – they may have guidelines for submitting files.
Even on my favorite operating system, OS X there are several methods available to users for making PDFs of documents. These methods do not all create the same PDFs. (The difference is in the meta-data contained and in the size of the files.) This is pointed out by Rob Griffiths in Macworld in an article on privacy, and being aware of PDF meta-data which might transmit more personal information than the document creator might desire. However, what Rob points out is that there are several methods of producing PDFs on OS X and these various methods include or exclude various meta-data details. Just as privacy concerns might motivate the removal of embedded meta-data (or perhaps the creation of PDF without meta-data), the accuracy of archive quality should drive the inclusion of meta-data in PDF files hosted by archives. There are two obvious ways to increase the quality of a PDF in an archive:
- The individual can enrich the PDF with meta-data prior to submission (risking that the institution will strip the meta-data embedded and input their own meta-data values)
- The archive can systemically enrich the meta-data based on the other meta-data collected on the file while it is in their “custody”.
As individuals we can take responsibility for the first point. There are several open source tools for editing the embedded meta-data, one of these is pdfinfoAnother command line tool is ExifTool (Link to Wikipedia). ExifTool is more versatile, working with more file types than just PDF, but again this tool does not have a GUI.. I wish I could find a place to download this command line tool, but it only seems to be in linux software repositories. However, there are several other command line packages which incorporate this utility. One of these packages is xpdf. Xpdf is available under GPL for personal use from foolabs. The code has to be compiled from source but there are links to several other websites with compiled versions for various OSes. There is an OS package installer available from phg-online.de. For those of us who are strong believers in GUIs and loath the TUI (Text User Interface, or command line) there is a freely available GUI for pdfinfo from sybrex.com.
Because I use PDFs extensively in matters of linguistic research I thought that I would take look at several PDFs from a variety of sources. This would include:
- JSTOR: Steele (1976). JSTOR is well known archive in academic circles (especially the humanities).
- Project Muse: (Language) Ladefoged (2007). Project Muse is also another well known repository for the humanities. Langauge is a well respected journal in the linguistic sciences, published by the Linguistic Society of America.
- Cambridge Journals: (Journal of the International Phonetic Association) Olson, Mielke, Olson, Sanicas-Daguman, Pebley and Paterson (2010) Cambridge Press, of which Cambridge Journals is a part, is a major publisher of linguistic content in the English academic community.
- SIL Academic Publishing: Gardner and Merrifield (1990) This PDF is found through the SIL Bibliography, but prepared by Academic Publishing (department) of SIL.It is important to note that this work was made available through SIL’s Global Publishing Service (formerly Academic publishing) not through the Language and Culture Archives. This is evidenced by the acpub used in the URL for accessing the actual PDF:
www.sil.org/acpub/repository/24341.pdf. As a publishing service, this particular business unit of SIL is more apt to be aware of and use higher PDF standards like PDF/A in their workflows.
- SIL – Papua New Guinea: Barker and Lee (n.d.) but made available online in 2009 by SIL – Papua New Guinea.
- SIL Mexico Branch: Benito Apolinar Antonio, et al. MWP#9a and Benito Apolinar Antonio, et al. MWP#9b It is interesting to note that the production tool used to create the PDFs for the Mexico Branch Work Papers was XLingPaper. XLingPaper is a plugin for XMLMind, an XML editor. It is used for creating multiple products from a single XML data source. (In this case the data source is the linguistics paper.) However, advanced authoring tools like XLingPaper, LaTeX and its flavors like XeTeX should be able to handel assignment of keywords and meta-data on the creation fo the PDF.
- Example of a PDF made from Microsoft Word: Snider (2011)
- Example of a PDF made from Apple Pages: Paterson and Olson (2009)
The goal of the comparison is to look at a variety of PDF resources from a variety of locations and content handlers. I have included two linguistic journals, two repositories for journals, and several items from various SIL outlets. Additionally, I have included two different PDFs which were authored with popular wordprocessing applications. To view the PDFs and their meta-data I used Preview, a PDF and media viewer which ships with OS X, and is created by Apple. Naturally, the scope of the available meta-data to be viewed is limited to what Preview is programed to display. Adobe Acrobat Pro will display more meta-data fields in its meta-data editing interface.
- Project Muse:
- Cambridge Journals:
- SIL Academic Publishing (not the archive):
Among the PDFs surveyed Academic Publishing was the only producer to use Keywords. They were also the only one to use or embed the ISO 639-3 code of the subject language of the item.
- SIL – Papua New Guinea:
- SIL Mexico Branch:
Work Papers #9a
Work Papers #9b
- MS Word Example:
Notice that in the title that the application used to create the PDF inserts “Microsoft Word – ” Before the document title.
- Apple Pages Example:
As we can see from the images presented here there is not a wide spread adoption of a systematic process on the part of:
- or on the part of developers of writing utilities, like MS Word, or XLingPaper, to encourage the enduser to produce enriched PDFs.
- Additionally, there is not a systemic process used by content providers to enrich content produced by publishers.
However, enriched content (PDFs) is used by a variety of PDF management applications and citation management software. That is, consumers do benefit from the enriched state of PDFs and consumers are looking for these featuresThe discussion on Yep 2’s forums high-lights this point. Yep 2 is a consumer / Desktop media & PDF management tool. There are several other tools out there like Papers2, Mendeley, Zotero even Endnote..
If I were to extend this research I would look at PDFs from more content providers. I would look for a PDF from an Open Access Repository like the Rugters Optimality Archive, a Dissertation from ProQuest, I would also look for some content from a reputable archive like PARADISEC, and something from a DSpace implementationXpdf can be used in conjunction with DSpace, in fact it is even mentioned in the manual..