Q: We have been using PDFTron for a few years and are very happy with it.
Is it possible using PDFTron to merge several digitally signed PDF Documents in to a single document without invalidating the signatures contained in each one?
Will the PDFDoc.e_Incremetal flag passed to PDFDoc::Save(…) do that?
You can append data (e.g. another digital signature) to a previously signed document without invalidating the existing certificate using e_incremental flag (in PDFDoc.Save()).
However this technique would not work if you need to merge multiple signed documents, since one of them would be invalidated. A potential technique would be to use PDF Portfolios (a.k.a. PDF Package) where multiple PDFs are wrapped in a PDF package that could also be signed.
For example of how to create and consume PDF Packages using PDFNet please take a look at PDFPackage sample: