If you open a PDF document and your viewer displays a panel (like you see below) indicating that

  1. the document is signed by invoicing@amazon.de and
  2. the document has not been modified since the signature was applied You assume that the displayed content is precisely what invoicing@amazon.dehas created.

During recent research, we found out that this is not the case for almost all PDF Desktop Viewers and most Online Validation Services.

https://www.pdf-insecurity.org/