Please be more specific about which kind of PDFs you would like to read. You mentioned "physics" PDFs? I assure you, that with PDFs containing anything different from plain text (like graphics, plots, code or formulas) you will run into big problems with IVONA and similar txt2speech software.
Besides that: How important is portability for you? Do you plan on adding written or sketched annotations to your documents?
Yes, reading PDFs of any kind seems to be possible with 6 inch devices. I have seen a lot of people reading scientific documents on their Kindle and Sony PRS-T2. But I was always wondering, how they can endure this pain - they were always reading with tiny font sizes and only half a book page on screen at a time.
Reading PDFs on a 6 inch device never was an option for me. That said, I hate reading on backlit displays (LED, LCD and the like). So I bought the Boox M92 (from
www.ereader-store.de - ships internationally and very good reputation in the Onyx Boox subforum). It's really expensive and you can't say the software is fully developed. The software doesn't really use the hardware's great potential. But for pure PDF reading, it's absolutely fine.
By the way, "auto crop margins" is a nice feature and works pretty well in most cases, but in some cases, it's still the best idea to crop the margins on a PC using software like Scan Tailor or BRISS.