Improve PDF/A compliance

[florianbepunkt]

I had the need to implement Facture-x/ZUGFeRD compliant PDF for invoicing.

During this task I found two issues with PDFlib.

1. File attachments are not PDF/A3 compliant (maybe not PDF/A compliant at all).

When adding an attachment, the attachment needs an AFRelationship entry with one of the following values:

type Relationship = | 'Alternative'
      | 'Data'
      | 'Source'
      | 'Supplement'
      | 'Unspecified';

Also an AF entry in the catalog must be added.

Both points should not add issues for different subsets AFAIK, so the only harm would be an increased API surface when adding attachments (add an option to set the attachment relationship).

2. Font CIDSet generation

The CIDSet generation here

pdfkit/lib/font/embedded.js

Line 186 in 2554c08

if (this.document.subset) {

breaks PDF/A3 validation (probably A2 as well). It is only needed for A1 AFAIK, so it can be omitted for other subsets, which would make the resulting PDF compliant.

I have both features implemented in a downstream repo of React-pdf (which uses a fork), but I can port those changes and draft a PR, if this is worth merging. Both points are small in scope and should be backwards compatible.

[blikblum]

I have both features implemented in a downstream repo of React-pdf (which uses a fork), but I can port those changes and draft a PR, if this is worth merging. Both points are small in scope and should be backwards compatible.

Please open a PR with only these changes