I don't know if the library software is capable of doing this, but maybe it is.
But I really meant opening the file in a word processing program. MS Word or openoffice should do the trick.
If you have a pdf, you can just select all the text and then dump the text into a document. Then you can change the font and size to your needs.
After doing so, just print out the document using something like freepdf or so and it will create a pdf with the adjusted font and size.
You can either connect the reader to the pc and get the files from there or just use a card reader. What I would do though is making a copy of the file you want to change first. This way the original keeps intact in case the output does not meet your expectation. There's some trial and error in this, but as soon as you have one document adjusted so that it fulfills your requirements, you know the settings an can apply them to other documents, too.
I hope I could help
Have a great day.