Tuning tablet into e-book reader? Since I often need to read PDFs, I figured I should get an e-book reader from eg. Kobo or Pocketbook, but I was told it's not a good idea because e-book readers have a tough time rendering PDFs.

Alternatively, are there good tablets and hardware/software that can reduce the amount of power used by the screen and somehow make the the screen easier to read?

Thank you.
Aside from the usual battery-saving tips (like disabling data / Wi-Fi), you could use an app like Screen Filter to reduce the brightness of the device; this reduces power consumption. 

If available, you might also invert the colors in your reader app. For example, ezPDF Reader has a Night mode feature which makes the background black and the text white (as opposed to the standard black on white). Of course, this is only applicable to PDFs with actual next, not scanned images.