Permission to view/edit document in Pages on my iPad

I keep getting periodic messages from Pages on my iPad when I try to open documents that I have created and are located in various places on my iPad. Most often I am using a Dropbox to open my files. I get a message that says

The file “.......” couldn’t be opened because you don’t have permission to view it. You don’t have permission.

I hope somebody can help with this. It is extremely frustrating. The document opens in Word just fine, but not in Pages.