Which one is the owner to open a pdf file Acrobat X or Reader X? You can check this by double clicking any pdf file and whichever application launches pdf file (Acrobat X or Reader X). If it gets launch in Reader X then try once in sandboxing mode off and then try launching this pdf.
For disable sandbox mode off follow these steps
1. Launch Reader X
2. Go to Edit option -> Preferences-> General
3. There is an option Enable protected mode at start up (Highlighted in the image below)
4. Uncheck the checkbox and quit reader
5. Then try launching your pdf file