Yup, even I'd like to mention that not go for 7" for serious PDF reading. I have Nook Color & I learnt this lesson hard way...
Try an app called mantano. The free version has reflow & crop.
9.7 or 10.1 is ideal for reading
A4 size pdfs. 9.7 is actually a 5% larger screen than 10.1 so you can read in portrait orientation with it.
10.1 screen in landscape gives a 8.5 inch page width so A4 can be read without any modification. If you do not read A4 size pdfs then a smaller size screen will suffice. I had no choice but to get 10.1 as 90% of PDF's i read are A4 size.
If you need to remove margins and have smaller screens a more permanent cropping solution is to use a utility called k2pdfopt. Its free and very well made.
Thing with pdf's is it depends on the file size i find single core tablets tend to struggle, scroll and then wait for it to render etc.
Dual core is better. You need a minimum of 1GB RAM.
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Outdoors? eInk is the only option and their PDF support just sucks.
If you stick to kindles & nooks yes.
But there are other options like Onxy 9.7 M92, goes under the name Icarus excel in the US. double the price and can handle large pdfs (10-20MB) without issue. No touch screen though so i'm holding off until they get that.