PDF to PDF/A

Yes. Converting to PDF/A needs a server, so this file is sent over an encrypted connection. The copy there is deleted as soon as the result is sent back, unless you are signed in and asked to keep it.

PDF/A-2b suits most archives and supports transparency — the usual choice. Pick a "u" level (2u, 3u) if reliable text extraction matters more than accessibility, an "a" level (1a, 2a, 3a) if the file must work with assistive technology, and Part 3 (3b/3u/3a) if you need to embed file attachments. If an institution asked for a specific level, use that one — the conformance menu explains each as you pick it.

By default the conversion is refused with a clear reason rather than returning a file that looks right but fails validation. Turn on "Allow conformance downgrade" to let it retry at the next lower level instead — the result always says which level it actually achieved.

Yes. Rewriting a PDF for PDF/A invalidates any digital signature already on it. There is no way around that; sign the archival copy again afterwards if you need to.

Yes. It runs on a server rather than in your browser, so it has a cap of its own: 25 MB and 50 pages. A file over either is turned away before anything is sent.