Tau Phi when I click on the pdf I get a window that shows this:
Have you tried getting the PDF generator to produce a table of contents with the names of the letters and name for major sections like the Principal Doctrines, Will of Epicurus, etc? That would probably be more useful than the page thumbnails, if that's possible. And yes I agree with Bryan's suggestion and I will rework a section of the front page to make it easier to see where to find versions of the texts like this.