Also, what ygert said. Converting from ePub to mobi is easy with Calibre, and converting from html to ePub is easy with Web2FB2. Even doc, docx, and, rtf stand a chance of being converted to ePub with LibreOffice and the correct plug-in (and that’s if you can’t just increase the font size on everything and export as PDF). Furthermore, somebooks have PDF versions optimized for smartphone or e-ink reading; it’s pretty rare, but its a treat when it happens. More commonly, if you can find a Powerpoint presentation that covers the material you are interested in, that would be great; converting to PDF will leave it in a much more suitable aspect ratio.
Use K2pdfopt, briss, and/or soPdf.
Also, what ygert said. Converting from ePub to mobi is easy with Calibre, and converting from html to ePub is easy with Web2FB2. Even doc, docx, and, rtf stand a chance of being converted to ePub with LibreOffice and the correct plug-in (and that’s if you can’t just increase the font size on everything and export as PDF). Furthermore, some books have PDF versions optimized for smartphone or e-ink reading; it’s pretty rare, but its a treat when it happens. More commonly, if you can find a Powerpoint presentation that covers the material you are interested in, that would be great; converting to PDF will leave it in a much more suitable aspect ratio.
If all else fails, consider buying a Kindle DX.