PDF files from certain sources might be secured or DRM protected or encrypted. This might prevent you from adding annotations, editing, printing or even viewing the file. Essentially the file is read-only for you, locking you out of making changes to it.
A PDF file which is encrypted can be decrypted using QPDF:
$ qpdf --decrypt in.pdf out.pdf
Tried with: QPDF 3.0.2