Just another thought and some clarification...
Scribus, and InDesign support creation of form fields in a PDF when you are creating a document from scratch. PDFs are imported like a graphic image, and are un-editable in place. (There is an available commercial plug-in for InDesign that allows editing inside of the app- "PDF2ID", but I don't think it supports creation of form fields, itself. There is most likely a corresponding 'xTension' for Quark xPress that provides the same function)
The other thing that is a potential problem is the permissions (if any) set on the document. PDFs have two levels of permissions that may present problems. The lower level is 'document' security, which prevents printing or changes to the document, and there are plenty of free utilities to "fix" this. The second is 'owner' security, which encrypts the whole document, and controls opening, viewing, and changing the 'document' security setting; and unless you know the password, you will not get too far in editing it, or even printing it, if the 'document' settings prevent this. (it's fairly strong encryption, and even with good resources (the right software, multiple GPUs to handle the number crunching and password generation, and a comprehensive password list for a "dictionary attack" it's likely to take even a determined person a week or better to hack the password. A "brute force" attack with randomly determined character strings for the password would take even longer. Not totally un-crackable, but effectively so for most purposes. Check that the form you want to use has the security turned off before you even start trying to make changes like adding form fields (under 'File > Properties > Security'). Hope this helps.