A resolution is impossible at this time
Disclaimer: I am not versed in German or EU law particularly (but am familiar with the relevant law in other countries to various degrees.)
It appears that, so far, no-one has been prosecuted for individually downloading copyrighted material. This also suggests this matter, per se, has not been specifically ruled on by the German courts. (It may have been ruled on in passing as part of another legal matter, but other answers here have not quoted such a ruling).
Now, the German legal system is a "continental law" or "civil law" system, so precedents do not matter as much, but still - we do not have here anything like a trained judge, having heard best-effort arguments from legal professionals on both sides, making a ruling. Until that happens, I don't think @Allure's question will be resolved.
More specifically, there are considerations such as:
- Whether downloading (as opposed to uploading to someone elsea publicly-accessible location or uploading to another individual) constitutes making a copy of a work.
- How the law applies in a situation where copies have been circulated en-masse and are freely available regardless of a single additional reproduction.
- Whether posting academic work onto the web for promoting academic studies, without license, is obviously-illegal (in the context of the personal use clause).
- Whether the personal use clause is not to be combined with Section 60c(2).
- Whether the inability to download "75% of the book" does not legalize the download of the entire book.
- Whether uploading/distribution made outside of Germany (and the EU), that doesn't target Germans in particular, can be faulted for not adhering to German law just because a website is also visible from Germany.
- Whether it is constitutional to prevent a person from what is essentially very much like "picking up a scientific study book and reading it". Forbidding this might violate constitution article...
- 1(1), human dignity?
- 2(1), free development of personality?
- 5(3), free science and research?
- et cetera, et cetera.
which have not been presented and ruled on in a court of law, in this context. And - perhaps for good reason.