Not able to annotate or mark up a percentage of .pdf files

There seems to be certain .pdf files that I’m not able to mark up nor annotate. I have a bunch of research papers in my database, but it looks like about 20% of them I can zoom, scroll, etc. but can’t alter. I can’t find anything so far that distinguishes the papers that I can’t touch vs. the ones I can. Using Preview, there’s no problem. Has anyone else encounter this problem?


OK, it might be on Preview. For certain pdf files, I can’t seem to “Save” with annotations, while other pdf files can. Once I did “Save As” after some annotations, I am able to alter that file in DevonThink. Perplexing.

Is the original file protected in some way?