PDF corrupted when opening in external editor

I just imported a bunch of PDF documents into the Scrivener project for research.
However, as I am trying to open the files in external editors (I’m using PDF Expert.), it failed and showed the following message:

But in the Scrivener project, as I clicked the document, it can still be shown:

How can I fix this problem? It was not always the case. One of the files (the first PDF that I edited) works fine. But later the others seem all corrupted (but I didn’t check them all). Thanks!

Different PDF viewers may have different capabilities. There might be something about this PDF that the macOS PDF Kit (which we use to display them) handles fine, but whatever PDF Expert is using doesn’t like it. You could try Skim, a free editor that uses the same engine we do.