I receive PDFs from a publisher that I never have problems opening, adding comments or adding edit marks using Adobe Acrobat X Pro. I send these commented PDFs on to assorted colleagues for additional comments, not doing anything special with saving. One colleague switched to Adobe Acrobat DC Pro last month and now can not open any of these publisher commented PDFs any longer. They get message PDF is signed. I don't sign them, add security, nada. Looking at old discussion How do I edit a signed PDF document (Edit PDF) I am now doing a "Save a Copy" with no security, etc. to help this person access the PDF.
Why is my colleague unable to read these PDFs that no one else is having problems with?
Maybe it's doing more checks to protect the user from bad things. Reader Enabled files should not be edited Because it breaks the Reader Enabling. Why are they enabled at all?