Anyone good with gimp?

If it’s a « real » pdf consisting of text + markups + pictures you may be able to open it in LibreOffice Draw, and modify it, and still save it in pdf…

If it’s a « bad » pdf only consisting of pictures, you can open it in Gimp, choose resolution, pages to open, if you want one page per layer…

Or you could try MyPaint thru a terminal install? Look at this: http://mypaint.org/about/