Epub or similar formats like mobi or azw3 are the way to go, most PDFs are laid out for A4 or 8.5"×11" paper and the text will look very small. I've gotten most of my ebooks from zlibrary, soulseek is another place that's worth looking. Converting PDFs to epub is possible if the PDF has text that can be parsed by the computer, but the formatting will likely get fucked up to some degree. If the PDF is purely image-based than it can't be done.

For online articles, I would maybe look into the Firefox Pocket integration that Kobo ereaders come with, you can send articles from your phone or PC to your ereader over the internet. I have a Kobo but haven't tried this yet because I keep mine offline. Most ereaders come with a basic browser but I wouldn't recommend it for practical use because of the low refresh rate and slow processor.