We've found that we can store a PDF with an e-signature by keeping the PDF (Electronic File) attached. However, most of our user's settings are set to remove the electronic file (Keep original PDF files is off) in their options.

When they import the file under those settings the E-Signature is dropped/removed. 

My work-around right now is to have them enable "Keep original PDF file" so the PDF uploads. I'm just curious if there's another method/way to get E-Signatures to keep?