[Tarana]

Here are my thoughts. Since your primary need is to be able to read in sunlight, you are restricted to ereader devices completely (no tablets). Assuming that you have neither the time nor the programs to convert pdfs (calibre does a crappy job on this), you are probably better off with a Kindle. Why? Unlike Nook, you can actually magnify pdfs on a kindle to make the text or diagrams/illustrations bigger. Now, this doesn't necessarily make reading easier since you have to move the page from side to side, but at least you can read difficult to read text. Your bigger problem is the size of the documents. PDFs are much larger in size and would fill up any device a lot faster than either mobi or epub. The older Nook had an option to add a mini-SD card for this, the current version has no such option (although it has more space than previously). If your PDFs have a lot of illustrations, it will be even more difficult.

Now, I did my tests on a Nook Glo and two older Kindles (keyboard and 6"). Someone here with a paperwhite could confirm that magnification option, but I see no reason why they would take that out. There are a variety of lighting options if you don't want to spend the money on a paperwhite. The easiest is to buy a cover that lets you add a mini-light clip on. Both my sisters actually prefer that, while my Mom prefers the light in the Paperwhite. You absolutely want a cover to protect your device while traveling.

If you have the time and money and have mostly all -text pdfs, consider AABBY Transformer. It isn't as good as AABBY Finereader, but well above in capability than the PDF converter in Calibre. The program keeps coming on sale for around $40. I normally use my Nook HD+ for all my pdf reading (magazines, knitting books, cookbooks) because it simply is better suited for them with the larger screen. But no way you can read in sunlight with a tablet, even with a non-glare screen.

Note: I have no personal experience with Kobo.