Depending on the type and origin of the XFDF or FDF, one of the following methods should be used:

1. Replace all annotations in the PDF document with the annotations from XFDF or FDF using FDFUpdate API.
2. Add all annotations from XFDF or FDF to the PDF document using FDFMerge API (potentially introducing duplicates).
3. Merge XFDF directly into the PDF document using MergeXFDF API (introduced in PDFNet 9.2)

• In order for the API to function, annotations in XFDF should be named (see below)
• MergeXFDF API is the recommended solution for command XFDF files

See below, which API is best suited to the given use case.

This is a 2-step process: first XFDF needs to be converted into FDF, then FDF needs to be passed to FDFUpdate API. As a result of the import operation, the PDF document will have all annotations from XFDF and no other:

1. Annotations that are in XFDF only will be added
2. Annotations that are both in XFDF and PDF will be modified as needed
3. Annotations that are only in PDF and not in XFDF will be removed

FDFUpdate API works best for the scenarios like restoring annotations from a backup. Also it would be a good option when bringing in annotations creating in WebViewer, i.e. use FDFUpdate to import XFDF written by WebViewer.

// Import annotations from XFDF to FDF
FDFDoc fdf_doc = new FDFDoc(FDFDoc.CreateFromXFDF(xfdf_filename));

// Optionally read XFDF from a string
FDFDoc fdf_doc = new FDFDoc(FDFDoc.CreateFromXFDF(xfdf_string));

// Merge FDF data into PDF doc
PDFDoc doc = new PDFDoc(filename);
doc.FDFUpdate(fdf_doc);

This is a 2-step process: first XFDF needs to be converted into FDF, then FDF needs to be passed to FDFMerge API. As a result of the import operation, all the XFDF annotations will be added to the PDF document:

1. Annotations that are in XFDF only will be added
2. Annotations that are both in XFDF and PDF will also be added, i.e. duplicating those annotations
3. Annotations that are only in PDF and not in XFDF will not be modified

FDFMerge API works best for bringing new annotations into a document from a partial XFDF and for adding annotations to an empty PDF document.

// Import annotations from XFDF to FDF
FDFDoc fdf_doc = new FDFDoc(FDFDoc.CreateFromXFDF(xfdf_filename));

// Optionally read XFDF from a string
FDFDoc fdf_doc = new FDFDoc(FDFDoc.CreateFromXFDF(xfdf_string));

// Merge FDF data into PDF doc
PDFDoc doc = new PDFDoc(filename);
doc.FDFMerge(fdf_doc);

This is a 1-step process: XFDF is passed directly to MergeXFDF API. The API can both add and modify the annotations, but not delete them (except when using a command file). :

1. Annotations that are in XFDF only will be added
2. Annotations that are both in XFDF and PDF will be modified (see below)
3. Annotations that are only in PDF and not in XFDF will not be modified

MergeXFDF is also the API that should be used with command files (a flavor of XFDF with add-delete-modify instructions, see GetAnnotCommand() or AnnotationManager.exportAnnotCommand in WebViewer). In case of command XFDF, all operations specified by the command file, including deletions will be performed on the document.

PDFDoc doc = new PDFDoc(filename);
doc.MergeXFDF(xfdf_filename);

// Optionally read XFDF from a string
doc.MergeXFDF(xfdf_string);

PDF Form fill and form data extraction
Full sample code which illustrates basic FDF merge/extract functionality and full support for FDF (Forms Data Format)