@
kmsitsee,
So... you bought an E-Ink Kindle for mathematical texts, you claim you can't afford Kindle books (but you can afford to buy them elsewhere

) and you've never heard of
calibre for converting your EPUBs to AZW3?
If you are referring to PDF, I've got bad new for you.
You should've gotten a tablet -- E-Ink ereaders of all stripes tend to be a miserable reading experience for PDF.
The E-Ink Kindles are certainly not an
exception to this rule.
But you may want to try
k2pdfopt, which can help reflow a PDF to be easier to read on E-Ink devices like the Kindle (its original target audience).