Document accessibility extends the same core principles used on the web, meaningful structure, text alternatives, sufficient contrast, logical reading order, to files people download and open in other software: Word documents, PowerPoint decks, PDFs, and spreadsheets. Each file format has its own accessibility checker and its own quirks for how structure gets encoded.
Organizations that have invested heavily in website accessibility sometimes overlook the documents linked from that same website, annual reports, policy PDFs, downloadable forms, which can undo much of that accessibility work if a user cannot actually open and use what the site links them to.