Welcome to the forum, gthalas.
You can certainly read greek text-based pdfs on a sony, though it might not look pretty when you use reflow.
If you have an epub without embedded fonts, as long as it is not drmed, you can either embed a greek font like in my example above (you can use sigil for that), or, if you want to do it for more ebooks, you can place a greek font inside your reader and just alter the css to link to this font. Here is an example of how to do it:
https://www.mobileread.com/forums/sho...d.php?t=102146
More work than it should be, certainly. Ideally you should just place an ebook into your reader, and start reading, instead of all this tinkering... There are readers that can do that of course, and it's one of them I will be buying next.