XFA

XFA (XML Forms Architecture) is an alternative form technology Adobe layered on top of PDF, where the form is described in XML rather than as native PDF objects. It was built for complex, dynamic forms, the kind that grow extra rows, recalculate totals, or change layout based on what you enter.

The catch is compatibility. XFA forms generally need Adobe's own software to work properly and often break or show a warning page in browsers, mobile viewers, and most third-party tools. Adobe itself has deprecated XFA, and the PDF 2.0 standard dropped it entirely, so it is a technology on the way out.

If you run into an XFA form that will not open, the usual fix is to flatten or convert it into a standard AcroForm or a plain PDF, which restores the broad compatibility PDF is known for.