Is the toUnicode-cmap editable?

[Smogshaik]

This may sound very specific and advanced, but I really don't know much about PDFs yet.

What I want to do: take an existing PDF and export it in Adobe Acrobat Pro DC conforming to PDF/A2u standards. These are standards for the long-term archiving of PDFs. No experience with these standards is necessary to answer my question though, I think.

Some PDFs are unable to be exported that way because they have some toUnicode mappings that don't conform to the standards. More specifically: "'ToUnicode'-cmap contains zero as a Unicode value". This is not a huge issue I reckon, but I'd still like my PDFs to conform to the standard at the end.

Is there any way to access these mappings? I imagine them as a simple dictionary with key-value pairs of glyphs and Unicode values. As such, it should be easy to change. I can't find anything in the Acrobat though.

Can anyone help me with this please?

[Test Screen Name]

In PDF internals, a ToUnicode map is a text stream embedded in the PDF. Almost always compressed so it will not be simply editable or readable without using software that can decode PDF structures. 

[Smogshaik]

Thank you for the reply! Do you happen to know of any software that can decode those structures? I would assume only Adobe software can do that truly well. Since I'm a bit of a programmer, I'm keen on learning more!

[Test Screen Name]

The format of PDF is not a secret. It's described in this 1000 page book: https://www.adobe.com/content/dam/acom/en/devnet/pdf/PDF32000_2008.pdf

[Smogshaik]

While I'm thankful for this and certainly will look into it, this does not directly answer the question if there is a reliable way/software for uncompressing and editing the text streams within a pdf. Maybe reading the document will help, though. I've also ordered a book from one of the devs at Adobe who worked on the format.

[Test Screen Name]

You might look into the tool PDF Can Opener.