PDF-XChange Viewer (now superseded by the PDF-XChange Editor) is a freemium PDF reader for Microsoft Windows.

It supports saving PDF forms (AcroForms) and importing or exporting form data in FDF/XFDF format.

Since version 2.5, there has been partial support for XFA, and exporting form data in XML Data Package (XDP) or XML format.

[2] The following programming languages are supported: C++, C#, C, Visual Basic (classic), Visual Basic (modern), Delphi, and Clarion.

Its viewer is compatible with Wine, which provides another way to annotate PDFs on Linux.