Is this an issue with a particular PDF file or with all the PDFs? Please try with a different PDF file and check. If the file is stored on a shared network/drive, please download it to your computer first and then try again.
Also try to repair the installation from the help menu and reboot the computer and check.
You may also try to turn off the protected mode for testing. Go to Edit > Preferences > Enhanced Security > Uncheck 'Enable Protected mode at startup', Turn off the protected mode and uncheck Enhanced Security > click ok and reboot the application. Note: Please turn on the security after testing to avoid any security risk.