Originally Posted by fjtorres
PDf files are not ebooks but rather digital paper. As such the formatting is hardwired and controlled solely by the publisher.
Postscript, which is the decription language used inside PDF files, is turing complete.
Many text PDFs also contain the text as a separate content and the font as separate. Those that have fonts converted to paths, do not have text content as separate from the typeface.
So yes, the font is changeable if one uses a postscript interpreting system like ghostscript or similar.
This is also how reflow in PDFs works, changing the values in the postscript description.
So, for many if not mosts files the font is theoretically changeable as long as the PDF container is not protected and the fonts have not been converted to paths.
As for the PocketBook, I do not think it currently offers this functionality.