PDF/UA improvements

[MerlijnWajer]

VeraPDF now supports PDF/UA verification:

~/verapdf/verapdf --format xml --flavour ua1 /tmp/test.pdf  > /tmp/out.xml

We should fix the problems that it finds with our PDFs, I suspect that this will also help with the problems that Adobe finds.

This means at least:

• Adding the Primary language
• Mark Figure's as Artifacts
• Adding alt text to Figure's (we might not need to if we mark them as artifact)
• Define language for text blocks
• Potentially indicate the reading order?

[MerlijnWajer]

On alt text: https://stackoverflow.com/questions/34036200/add-alternative-text-for-an-image-in-tagged-pdf-pdf-ua-using-itext

The PDF spec chapter 10.6 and 10.7 are relevant: https://ghostscript.com/~robin/pdf_reference17.pdf

[MerlijnWajer]

The natural language for text blocks could be determined from the hOCR lang attributes, perhaps

[MerlijnWajer]

https://blog.adobe.com/en/publish/2017/07/25/accessible-pdfs-in-acrobat-dc-tagging-content-as-an-artifact.html
https://www.uottawa.ca/respect/sites/www.uottawa.ca.respect/files/fss-fixing-accessibility-errors-in-pdfs.pdf