This is the first instalment of a 2-part blog. It was prompted by the upcoming Digital Preservation Coalition briefing When is a PDF not a PDF?, for which I was asked to prepare a presentation. My initial idea was to give an overview of the work we did on PDF preservation risk assessment using a PDF/A validator in the SCAPE project. Most of this has already been covered by a series of earlier blog posts. Those blogs very much represent different stages of a work in progress, and I think this makes them somewhat challenging for readers who are new to the subject.

The purpose of this 2-part blog is twofold: first it is an attempt to give an accessible overview of the earlier work on PDF preservation risks, stressing the importance of PDF/A validator tools in detecting these risks. Second, it provides some tentative suggestions of how the ongoing work on the new VeraPDF PDF/A validator could close some of the gaps and limitations of the SCAPE work.

Preservation risks of PDF

The PDF format has a number of features that don’t sit well with the aims of long-term preservation and accessibility. This includes encryption and password protection, external dependencies (e.g. fonts that are not embedded in a document), and reliance on external software. More details can be found in the PDF entry of the OPF File Format Risk Registry. Below are some examples; I included download links, so you can try them out for yourself.

Document Open password

If you try to open file encryption_openpassword.pdf in Adobe Acrobat, you end up with this dialog:

Without the password, the file cannot be opened at all.

Print password

File encryption_noprinting.pdf can be opened normally, but you cannot print it:

Embedded Quicktime movie

File embedded_video_quicktime.pdf contains multimedia content in Quicktime format. Acrobat cannot render this format natively, and relies on an external player. This is what happened when I opened the file on my PC:

After I clicked on Get Media Player, I was taken here:

I wasn’t able to configure Acrobat to use a media player that supports Quicktime ¹.

External reference to multimedia file

The file movie.pdf contains references to external multimedia files. If you click on any of them you get an error like this one:

Font not embedded

File calistoMTNoFontsEmbedded.pdf uses Calisto MT, but the font is not embedded. Since Calisto MT is a Windows system font, the file looks fine on my Windows PC:

The font does not come pre-installed with common Linux distros, and as a result the file looks quite a bit different on my Linux machine:

3D content

The file digitally_signed_3D_Portfolio.pdf contains 3D artwork. Acrobat correctly renders the 3D content, which can be manipulated interactively by the user:

However, Acrobat aside, the majority of PDF readers don’t support 3D content, with the result that in other readers you may end up with something like this:

Detecting risky features

Archives or libraries may want to check their PDFs for one or more features like those shown above. Reasons for doing so include:

• Pre-ingest checks against an institutional policy (e.g. an archive may not accept PDFs that are password protected)

• Profiling of existing collections for preservation risks (e.g. embedded multimedia content in hard-to-render formats)

For this quite a few useful software tools are already available. For example, qpdf gives detailed information about encryption and password protection:

Similarly, the pdffonts tool that is part of xpdf is useful for checking whether fonts in a PDF are embedded:

As the number of features you want to check for increases, this approach becomes increasingly cumbersome: most of tools only cover some features, so you rapidly end up having to deal with a multitude of software tools and output formats. So you may ask yourself if there’s a way to do this more efficiently.

PDF/A validation

This is where PDF/A enters the picture. The PDF/A standards are nothing more than a set of profiles that impose some restrictions on a PDF, ruling out features that are not well-suited to long-term accessibility. Unsurprisingly, these include the very same features that we are interested in here, such as encryption, non-embedded fonts, multimedia content, and so on. Several tools exist that compare a PDF against PDF/A and report any deviations. These PDF/A validators are typically used to verify “true” PDF/A files; however, they can also be used to detect user-specified risky features in regular PDFs.

The professional version of Adobe Acrobat has a PDF/A validator built into its Preflight tool. After opening a PDF in Acrobat, it allows you to verify its compliance with a number of profiles, including PDF/A (currently A-1, 2 and 3):

This results in output as shown here:

This PDF² (which isn’t a PDF/A) violates the PDF/A-1a profile in several ways, but supposing we’re only interested in encryption and non-embedded fonts, the relevant information can be extracted from Preflight’s output quite easily. This example demonstrates the overall feasibility of identifying preservation risks with a PDF/A validator, but it is not scalabe to situations where you need to verify large volumes of PDFs. This will be the main focus of the second part of this blog series.