I bought a textbook from my school online. it is a pdf file that requires adobe acrobat reader dc on my mac. When I try to open it on adobe acrobat on my ipad, I can't sign in like I did on the computer. There's no option for preferences for me to allow myself to put in a username and password to unlock the document.
Mobile Acrobat has limited feature set compared with the desktop version. Many features available in desktop version are not available in Mobile. Encryption, I think, is one of those.