Originally there were AcroForms, part of the PDF spec. That's supported by pretty much everyone. Then, Adobe saw an opportunity make some enterprise money and

Originally there were AcroForms, part of the PDF spec. That's supported by pretty much everyone. Then, Adobe saw an opportunity make some enterprise money and introduced XFA (XML Forms Architecture). The XFA spec is associated with the PDF spec, and you can put XFA content inside of a PDF, but the XFA spec is actually larger than the PDF spec. It's utterly insane. I know that Adobe of course supports XFA, and there are various enterprisey things that support it to varying degrees, but I don't think it's well-supported by anyone outside of Adobe's implementation. Not only is it huge and complicated, but it also requires pulling in a JavaScript interpreter, which is a big ask for a feature that only exists because at one time Adobe thought they could turn it into another revenue stream. It's noteworthy that the PDF 2.0 spec specifically says that XFA is not only deprecated, but that any PDF which contains XFA content is considered out of spec for PDF 2.0. 2.0 goes back to AcroForms for all of that stuff and ditches XFA entirely. Likewise for JavaScript in general. Anything with JavaScript dependencies in PDF 1.7 is verboten in 2.0. In general, if you get stuck with a PDF with XFA content (not AcroForms), your best bet is to just use Adobe Reader to fill it out. Hopefully PDF 2.0 will take over the world eventually and everybody will be 100% back on AcroForms.