I’ve been working with the forms available from the Copyright Office, and while they can be filled out in Acrobat Reader, they cannot be saved unless you have a full version of Acrobat.
Or so the Copyright Office assumes.
New forms with special bar codes that can be scanned are forthcoming, but if you want to back up the forms you fill out, you can instead print the PDF to another PDF.
On a totally different endeavor, I downloaded PDFCreator, a PDF creation freeware. I experimented with creating PDFs from scores made in Sibelius. I got some great results, and I wondered whether the same can be done from other PDFs. So I printed my filled-out copyright forms to PDFCreator, and lo and behold, backups!
It’s a nice temporary solution till the Copyright Office makes its new forms available.