Why is my customer filter leaking memory on iOS?

Q:

The app I’m working on need to be able to read PDF files using an homemade encryption system. PTFilter seems to fit my needs.

Everything is working just fine except the fact that, for some reason, my use of PDFNet is not memory efficient at all.
Scrolling through a big file full of images make the memory grow until there is no space left, resulting to a crash.
Doing so is necessary because the method is called on a secondary, platform-independent POSIX thread that does not come with its own top-level autorelease pool. This is true of all callbacks declared in a C (as opposed to Objective-C) header.