"Low memory" when viewing PDFs with Paperwhite

[Mcmac]

I got my Paperwhite not so long ago, and I'm pretty satisfied with it.
I have a little problem though. Whenever I view PDFs that are not vector based (so scans for instance) first of all the page turn is very slow, and secondly, after a couple of page turns a dialogue comes up saying
"Low memory.
The selected item could not be opened. Your Kindle doesn’t have enough free memory."
The PDFs are scans, though good quality and low file size, for instance 8mb for a 165 page doc.
Any way to solve this?

Thanks for your help!