MobiOffice lets you encrypt your PDF files on a file level, allowing no one to view the file, unless they have the password.
1. Open a MobiOffice PDF.
2. In View mode, tap the menu in the top right. Tap Protect.
3. Type and retype your password.
Remove the password
1. Open a MobiOffice PDF.
2. In View mode, tap the menu in the top right. Tap Unprotect.