Thank you so much both for your quick answers!
First off, I performed the tests suggested by erin_b1. The outcome was interesting, though I have a hard time to correctly interpret what I found.
- After creating a different user account and logging into it, nothing changed. So the outcome was entirely negative. The PDF contents were still disappearing when a certain threshold was reached. And my findings were almost the same as the ones reported by VikingOSX. Thank you, my friend, at least I have the confirmation that I am not the only one with this issue.
- Secondly, when I booted my MacBook in safe mode, the issue disappeared. Though I must say, the computer became very laggy, it seems that a certain graphics acceleration function gets disabled when starting up in safe mode.
But how to interpret these findings? I don’t think there are any graphic-related plugins I installed. I have Affinity Designer and Photo and the Finder extensions of the latter program. I also run a legacy version of Adobe Indesign CS 6 and associated software. But the issue affects both the Preview application and the internal PDF Reader of Safari, so I believe it must have its origin at a deeper system level.
Furthermore, the issue shows up with a clean install of El Capitan on an external drive, and zooming in on my Mac Mini gives me a crash under El Capitan. Since there is not even a dedicated feedback page for Preview, I don’t know where to report these bugs!
Once again, thank you very much! Any further ideas would be very appreciated … 🙂
Alex