My previous adviser has some visual impairments associated with age. Any advice on how to make paper prints of PDF articles more legible, and accessible?
I think you may have a problem with this. PDF's are specifically intended to look the way they were designed, regardless of what's displaying it.
You may be able to import it into another program and make changes in that program's format. This may be easy, or not, depending on the rights assigned to the PDFs you're dealing with, and the options used when it was saved originally.
there could be problem with PDF. Because visually impaired mostly have problems with text in columns - especially articles - so, if you have normal document (DOC, ODT ...) then do not use columns and use sans font like Arial, maybe bold, which is best readable one.
The needed lowest font size depends on your clients. It is also necessary to take care about the colors - the text should be high contrast - best is black on white.