@ziya2510 Maybe I'm not doing the steps right to reproduce.

Goto https://www.pdfdrive.com/territorial-conflicts-in-world-society-modern-systems-theory-international-relations-and-conflict-studies-new-international-relations-d156760696.html.

Right-click on "Download (PDF)" and choose "save linked content as".

In the Save as Dialog, the suggested filename is:

Territorial Conflicts in World Society_ Modern Systems Theory, International Relations and Conflict Studies (New International Relations) ( PDFDrive ).pdf

I change the filename to:

Territorial Conflicts in World Society_ Modern Systems Theory, International Relations and Conflict Studies (New International Relations) ( PDFDrive ) add some more text.pdf

and save.

Result: Opera saves with my the even-longer filename I gave it no problem.

Now, if I instead save it as just:

Territorial Conflicts in World Society_ Modern Systems Theory, International Relations and Conflict Studies (New International Relations) ( PDFDrive ).pdf

and then open the pdf locally (with ctrl + o in Opera), click the download button for the pdf on the page and change the filename to:

Territorial Conflicts in World Society_ Modern Systems Theory, International Relations and Conflict Studies (New International Relations) ( PDFDrive ) add some more text.pdf

, it saves just fine too.

Even if I change the filename to:

Territorial Conflicts in World Society. Modern Systems Theory

like in your video, that saves fine in Opera too.

When you said you tried again, did you mean with the link to the PDF I provided? If not, see if you can reproduce the issue with that one. If not, maybe it's something to do with how the school site serves the PDF, which might explain why I can't reproduce.