If you're having issues specific to one file, there may be a problem with the file itself.
A great first troubleshooting step is to see if you have the same problem in other PDF readers, like Adobe Acrobat or iBooks. If so, that's a pretty clear indicator that the file is corrupt.
If you are able to open this file in other readers then try creating a copy of the file in iAnnotate.
To do this tap on the orange tab. Select "open a copy" from the dropdown menu.
If the copy will not open either try importing a fresh copy of the document into iAnnotate.
If you are still experiencing issues please contact our support team at firstname.lastname@example.org for further assistance.