Three suggestions:

• When reading PDFs on a Kindle, rotate the kindle 90ºs . This will make the page display much larger, though you will have to click down to get to the second half of the page. It's really much better. (rotation is in the settings)
• the free app ( https://calibre-ebook.com/ ) can convert PDFs to e-books, this is a little hit or miss
• if you have about $250 to blow, the Kindel-DX (discontinued) can still be bought, it's basically a giant Kindel and great for viewing PDFs ( https://www.amazon.com/Kindle-DX-Wireless-Reader-3G-Global/dp/B002GYWHSQ )