Oh, that rating.

Nothing planned for that yet, but that's a good idea I'll write it down.

Indeed I misread. I'll look into it.

Then i upgrade java for the last version and now i have:
G:\Ubooquity 3>java -version
java version "1.8.0_411"

You have the latest version of Java 8.

You need at least Java 17 to run Ubooquity 3.

For Windows you can use this one (but other exists, any Java 17+ should be fine): 

https://adoptium.net/en-GB/temurin/releases/?os=windows&arch=x64

I have added a "user rating" column in the database in version 3 (just in case) so that one day user can rate their books, but I don't know if and when I'll implement this feature.

That's by design: shared folder must be distinct, one cannot contain another.

Thanks to the file you sent me I was able to reproduce the issue and fix it. 

The fix will be in the next minor verion (3.0.3). :)

Thanks for the bug report.

I just did a few changes to the PDF rendering part to fix another bug. So you can either wait for Ubooquity 3.0.3 that will include these changes to check if your issue is fixed, or (even better) send me some examples of PDF file that triggers this error so that I can investigate them directly. My email: tom at vaemendis dot net.

I could not find a PDF containing a jbig2 image in my collection (I seem to have lost the one I used to test my implementation a few years ago).

Would have one that could be shared (or sent to tom at vamendis.net) so that I can use it to reproduce the issue on my side ?

Nice !

About the workarounds, if you think about modifications that would make this kind of theme easier to make, let me know.

I can't make any promises, but if some easy to make modification to Ubooquity's code can help, I might as well throw it in.