I don't know any Javascript library, but you can use FDF / XFDF files to do something like this. The idea is that you basically link your PDF file with an external FDF or XFDF file (XML version). When a PDF is uploaded, so is the FDF / XFDF file. You put the form fields in a PDF (which seems to already exist). They fill in this information and (if you add a button to the PDF form), click the button, which basically returns the POST to the server with these fields.
At this point, you can save them in a database, combine them with PDF, etc.
BTW - XFDF/FDF , . , PDF . PDF , , , .