The Kobo Aura HD and the T68 have the same size screen. I've been reading my PDFs on a Kobo with Koreader and have been pretty happy. They're mostly programming books so not a lot of diagrams but the diagrams I do have are usually pretty good. Occasionally they're a little small. I would think you'd be OK on the T68.
- The koreader software is supposed to run on Android but I've been frustrated to say the least trying to get it to run.
- Mantano reader has a great pdf mode that will automatically crop the pages, pretty much the same as koreader. When I'm using an Android device, that's my reader of choice.