Keynote document can't be opened although its size shows normally and how to repair it? I recently moved my some keynote documents into backup HD. And I found I can't open some of those someday. It showed that "xxx.key can't be opened",and then I just try open it in PDF preview, it showed that "It may be damaged or use a file format that Preview doesn’t recognize."   



But something weird was that those fail-to-open files remained normal size. Right now I'm not sure whether it is error of writing files in HD or keynote software itself.

Any other ideas as to how these documents can be repaired?
Try this:

Rename a copy of one of the broken presentation to have an .zip extension:
Bad_Presentation copy.key > Bad_Presentation copy.zip.

Then right-click on Bad_Presentation copy.zip and choose Archive Utility from the Open With> command. This will unpack the presentation into a folder named Bad_Presentation. Take this folder, give it a new name, (Bad_Presentation_Fix) then right-click and Compress the folder back to a .zip formatted file. Lastly, rename the resultant .zip file Bad_Presentation_Fix.key. See if you can open this in Keynote.

iWork documents are not what they seem. They are actually a set of bundled resources in a folder that looks like a single document in the Finder. Sometimes the bundle format can be messed up when transferring iWork documents into and out of file systems which cannot handle this format.

Good luck!