I have adobe acrobat 11. I use the back space key to edit an adobe acrobat PDF document by striking out a word. I then use what used to be called the "typewriter" and is now the add or edit text box option to suggest replacement text. Works fine. But I can no longer use the back space key to strike out a word. Sometimes, if I close the document and reopen it, I can use the back space key but not always, and, in any event, that is unbelieveably cumbersome. Closing the tools menu doesn't help, either: I still have the floating icon "A" telling me that adding text box option is still live.
Would request you to please check under the Tools 'Edit text & Images' is selected or not (like still 'Add text' should not selected),if option is selected you will be able to edit the document using the 'back space key'