Hi Jai,
This seems to be something related to the system configuration for PDF documents or the Adobe reader's configuration and there are a lot of notes to refer to on SAP Marketplace.
I suggest going through the notes by searching for the string "File does not begin with %Pdf" or the best can be, opening an OSS for this error or reaching out to your Basis team for support.
A few notes but more exist :
1146711 - Problem opening PDF files
/Hasan