We use the PDFTron product using C.
We have noticed that when we remove the metadata from a PDF file,
using the following code:
PDFDocInfo docInfo = m_pPDFDoc->GetDocInfo();
that data which has been input into form fields also gets removed.
Obviously we just want to remove the Author etc data and not the data
entered into the form fields.
using docInfo.SetAuthor(""); etc on their own does not seem enough as
some PDF apps (PDFXchange for example) still displays the Author etc
...presumably its being stored in more than one place within the PDF
Can provide before & after sample files if needed.