Your comments

Hi,
Ubooquity already preloads one page in advance. I could load a few more to be safe.
But the preloading mechanism does not work well on iOS, that's something I have to investigate.
Yes, I'll add an option for that.
Ok, I understand. You are talking about a real-time user activity monitoring, with quota management.
Although technically possible, it is beyond the scope of this project (a simple content sharing server), so it won"t be implemented.
I stand corrected. :)
Thanks for the information and the tests you have done !

I am a little bit reluctant to add almost 5 MB of additional jars in the Ubooquity binary for what I think to be quite specific cases, but I'll add an entry in the FAQ so that other people can easily solve this problem if they encounter it.

You can see these information in the log file.
Look for lines like:
20140726 18:10:46 [pool-2-thread-6] INFO  com.ubooquity.a - User successfuly logged in. [login:Tom][ip:10.0.0.5]

to know when a user logs on, and

20140812 19:57:34 [pool-2-thread-5] INFO  com.ubooquity.f.d - Served file to Tom in 21 sec: Deadpool 014 

to know when a user downloads a file.
The error you get comes from images contained in your PDF files that are encoded using the JPEG2000 format.

JPEG2000 is an image compression format that is quite efficient but also very complex and that never really took off. Because it is complex and not really used, it has very poor support in applications and libraries.
In your case, the PDF file is decoded by Ubooquity using the JPedal PDF library. This library does not support JPEG2000 out of the box and has to rely on another external library: Java Advanced Imaging (JAI). The problem is that JAI is an old library that is not maintained anymore (abandoned since 2006). 

So in theory, depending on the architecture you're running Ubooquity on, you might be able to download the last JAI package, install it and add the JAI library to your classpath when running Ubooquity. But I doubt it will succedd.

Bottom line: Ubooquity does not support JPEG2000, sorry.