When I render a PDF that is a dynamic XFA form, I see an error message: "Warning: This form is not supported with the current version of Acrobat or Acrobat Reader"

Dynamic XFA PDF forms contain backup placeholder content in addition to the XFA data describing the fields and content of the form. The placeholder content can vary but is typically a warning to upgrade or use Adobe Reader.  When a dynamic XFA PDF is opened by a viewer or application that does not support XFA, the application will typically display (or find) the placeholder content instead.  

APDFL Support for XFA

To process dynamic XFA documents with APDFL, you will need the optional PDF Forms Extension  add-on component.  The PDF forms Extension supports flattening, importing data and exporting data for traditional AcroForms, static XFA forms and dyamic XFA forms. See Form Types for a breakdown of PDF form types.

Note that to open any XFA documents, the application code must explicitly permit it:

PDPrefSetAllowOpeningXFA(true); // C++
lib.AllowOpeningXFA = true;     // .NET
lib.setAllowOpeningXFA(true);   // Java

PDF Forms Flattener

Workflows that need to process PDF forms but do not need the full APDFL SDK functionality should consider the PDF Forms Flattener.  The PDF Forms Flattener is a command line utility with equivalent flattening, importing and exporting functionality.