Use File Attachment annotation for the link to the embedded file

[Intelligent2013]

Source issue: https://github.com/metanorma/metanorma/issues/407#issuecomment-2321962832

For TC4, the type of PDF annotation generated is incorrect.

It is currently a Link annotation (/Subtype /Link supporting a URI via the /A (Action) key) whereas it should likely be a File Attachment annotation (/Subtype /FileAttachment with /FS entry to an embedded file object). Although technically an Embedded GoTo action is possible using the current Link annotation (by changing the /A' entry), I'm not sure how widely supported that feature is in browser-based PDF viewers and the less widely used PDF viewers.

This is how the Apache FOP is working (the source code https://github.com/apache/xmlgraphics-fop/blob/c11f43c9dbf529b87820d77ef501aa10699fc9d8/fop-core/src/main/java/org/apache/fop/pdf/PDFFactory.java#L699) Apache FOP add JavaScript action to open the embedded file:

PDFBox PDFDebugger:

Adobe Acrobat:

From PDF Reference 1.7:

To do:

• [ ] change /Subtype /Link to the File Attachment annotation (/Subtype /FileAttachment with /FS entry to an embedded file object

[Intelligent2013]

The FileAttachment annotation is working differently than text with hyperlink with Javascript function (current):

• the active area is icon (Graph, Paperclip, PushPin or Tag) vs. text hyperlink
• the attachment opens after double-click on the icon vs. just click on the text

Example from ISO_32000-2_sponsored-ec2.pdf with two 'paper clips' links:

Another very simple PDF with /Subtype /FileAttachment: structure-attached.pdf (found on GH somewhere, opening in the Adobe Acrobat only, not well-formed PDF)

Additional tasks:

• [ ] remove blue color and action from the link
• [ ] add icon after the link.

[Intelligent2013]

Current progress, proof-of-concept example: test_attachments.tc4.presentation.pdf

1. the Adobe Acrobat shows the message:

The double-click on the 'Paperclip' icon doesn't work until the user click on the button 'Enable editing'.

2. The 'Attachment' tab contains two files - attached file and 2nd one shows when annotation is created. I'll investigate how to omit the second one.

Note: We can't generate PDF/A-3 for BIPM brochure, because the link to the embedded .mml file realized in the Apache FOP via JavaScript function, that doesn't allow by PDF/A-3.

[petervwyatt]

That last file is a special file from Ange (a file format expert who experiments with minimal files)...

Other PDF files with file attachments on the public web:

• https://bugs.ghostscript.com/attachment.cgi?id=7736
• Isartor Test Suite for PDF/A - isartor-6-5-2-t02-fail-a.pdf (it's a valid PDF but a fail case 'cos earlier PDF/A versions did not allow file annotations)
• various PDFs from https://github.com/openpreserve/format-corpus/tree/master/pdfCabinetOfHorrors such as this one
• the veraPDF corpus (both pass and fail examples, depending on conformance level) - e.g. https://github.com/veraPDF/veraPDF-corpus/tree/staging/PDF_A-3b/6.8%20Embedded%20files

[petervwyatt]

@Intelligent2013

The double-click on the 'Paperclip' icon doesn't work until the user click on the button 'Enable editing'.

Correct. That is a PDF/A formal requirement to ensure that any user interaction (such as extracting an embedded file) cannot / does not mess with page content. This is not a bug :-)