Is there any restriction in storing files (audio, video, PDF, Word, all kinds...) on the device storage

Solution 1:

Apple does not scan the content of your local storage device. To do so would likely violate privacy and data protection laws in many jurisdictions.

Apple did announce that it planned to introduce a method of scanning photos that were uploaded to iCloud for known images of child abuse, but the scheme has been widely denounced by security professionals, privacy and rights advocates, and end users; and Apple has postponed the scheme's introduction.

If you have access to online academic journals that allow you to download PDFs, then keeping a copy for private use is not a breach of copyright. (Though you should satisfy yourself of that by reading the small print on the portal's pages.)