What is the android version of your device? Is it Android KitKat(4.4) and the file is located on the external SD card? Android KitKat has enforced some restrictions on the SD card access by third party applications: http://www.androidcentral.com/kitkat-sdcard-changes
If you are not running Android KitKat or if your file is not located on the external SD card, would it be possible for you to share the exact steps along with the device and OS details?
Do you also observe the dialog where you are asked to save a copy of the pdf file while opening the file from a read-only location? What happens when you save a copy? Are you able to edit the file normally?
Awaiting your response.