I think I found a fix to this. I discovered it because the documents do save updates, but the document previews don't appear to update - thus leading me to believe it's a problem with that phase of the saving process. By testing this locally, it seems to work on every file, but your experience may vary. Long story short, when you open the document:
- On new documents - be sure to uncheck the "Include preview in document" option when saving for the first time.
- On existing documents, the easiest work-around I could find is just to save a new copy of the document. Just go to File > Save As..., and again, make sure to uncheck the "Include preview in document" option before you save. Other than that, I couldn't find a way through the application to disable it from saving a preview of the document. Maybe it's somewhere obvious, and I just overlooked it... which is entirely possible/probable.
In either case, you may want to uncheck the "Include preview in document by default" option in the Preferences, so you don't have to remember for future files.
I hope this helps.
For the extra nerdy:
Since I couldn't find a way to disable the preview within the user interface of Pages, I dug into the package contents of the saved files. If you choose to go this route, there is a value in the un-gzipped "index.xml" file called "sl:SLSaveQuickLookPreviewProperty". The value is probably set to 1, but should be changed to 0 to make it stop creating previews/pdfs. You'll need to re-gzip that file to take the place of the "index.xml.gz" file in the package contents. You should be good to go. You might want to delete the existing pdf while in there, since they'll be useless/out-of-date going forward. If all of that sounds foreign to you, just pretend I didn't write this paragraph. Even if you did understand it, it's probably easier to just save a new copy. I'm sure somebody with more skills/interest than I could make a quick file patcher that makes this change in whatever file you drop on it.