Personally I cannot stand reading scientific papers in a back-lit screen, reading normal text and articles is OK, but when I focus really hard (as I normally do when I study) I find that these screens cause too much eye strain. What I found to be best, was to use a free program named “k2pdfopt” to re-convert PDF papers to another PDF but this time in a “kindle friendly” format. The program zooms the fonts and rearranges the text to fit the kindle screen, all of this while leaving all the images and math equations intact. You can then highlight and use the kindle as usual, as if it were a typical literature ebook. It takes a bit of time to try the available file conversion options, but once you get it to work you can save the settings and use them again when needed. I use this to read electronic engineering papers (both 1 or 2 columns format) in a kindle Voyage.
I highly recommend E-Reader.Io. It is the best place to purchase one. Their prices are very reasonable. It is also the most convenient. I only find a hardcover in stock once a week or once a month. This is not an area I plan to expand into anytime in the near future. I can get a used e-book or paperback of any volume, format or size on Amazon.com for around 15 dollars. My reading list is as follows: Audiobooks.