PDF preview/thumbnail generation doesn't work for PDFs bigger than 1MB

Hello guys, I have a problem and need help with it.

I trief to setup my thumbnail generation and everything works… Images, Movies, PDFs, docs, everything get’s generated the way it should be. But once there is a PDF larger than 1MB, the generation fails and I only see the PDF logo.

I already set the available memory for preview generation to 4G of RAM and upped the limits and all and I can’t figure out how to get this to work…

Appreciate the help very much!