Kindle paperwhite 7th gen - can't get my books into kindle!

I have several books in PDF format on my laptop but the text is too small if I just add them directly to my Kindle.

I tried converting to .epub and sending it via email and through amazon websites "Send to Kindle", it just gives me an error message that the file can't be added, and it doesn't say why...

I tried through Calibre but I'm not sure if I'm doing it correctly.
Not sure what to do now, if anyone could help me I'd be very grateful!