Why can't I add and remove pages from some pdfs using Preview?

The functionality in Preview to add and remove pdfs seems to work fine on some pdfs and not at all on others. I tested this by creating a pdf from a simple Word doc. I exported a page of it to create a new file, which worked fine.

Using both the Edit>Insert method and the dragging thumbnail method, I am able to add additional pages to the original pdf file I created. When I try the same actions with the second (exported) file, the options are greyed out and dragging the pages across doesn't work.

I then tried inserting pages into a random pdf that I did not create, and I was able to do that with no problems, though this doesn't always seem to be the case.