It's a software problem, surely. And the problem is Acrobat reader.
In fact, even on my PC, I find that the rendering of text isn't so good compared to PDF X-change viewer.
Try it on a PDF file, you will see that clearly. The text doesn't look the same and is easier to read on PDF X-change viewer.
I think that somehow the eink screen of Pocketbook only increases this problem, but isn't the real cause of the strange rendering of PDF files.
For example, look on Epub, other format or even some types of PDF files. It's very clear, without problem to read on my Pocketbook!
I wish that they abandoned Acrobat Reader. How good would it be with a Pocketbook version of PDF Xchange viewer !