My personal opinion is that the use of PDFs for web forms to a general audience is now a less good prospect than it ever was, because we have to deal with such a maze of non-compliant apps including many browsers, and the PDF viewers which come with Windows 8 and Mac OS, and on tablets and phones. Some of these ignore forms, the worst appear to fill forms but leave them corrupt. Hand-holding your users through a process they will not enjoy of getting Adobe Reader going AT ALL can be a major drain on resources (and it's tablet/phone support still leaves much to be desired). This is a deeply ironic side effect of the success of PDF and Adobe software.
I suggest you look very hard at using HTML forms for the filling. Of course, you can generate and return a PDF, that should work very well. Signing is a very complex issue, however.
Of course if you control the user environment everything changes and PDF form based solutions can be ideal. This would include getting Acrobat for each user.
