What error message are they reporting back to you?
Are you by any chance enabling password protection?
When you scan a document you will get images of the pages. Acrobat can't edit images.
Actually, it's not that simple anymore: When you have Adobe Acrobat DC, and try to edit a scanned document, it will actually perform OCR and prepare the current page for editing, You will see something like this:
If you send a document - before you make any edits - to somebody who has an older version of Acrobat, they will not be able to edit the file without running OCR. If they have Acrobat and are able to edit the file, then they also should have access to the OCR feature in Acrobat. You can of course OCR the document first, before you send it out (Tools>Enhance Scans>Recognize Text). Make sure that you select "Editable Text and Images" on the OCR settings dialog - otherwise you end up with text that can be selected and extracted, but you still have an image that cannot be edited.