Hi Sean, welcome to MobileRead!
If you have general questions to reading technical pdfs on an eink reader, please scroll down until the "Similar threads" section or use our search function. This topic has been discussed here many times, and yes, a 9.7" device or bigger is the only sensible option.
As far as I know you can't buy Onyx devices in the USA. eReader store
http://ereader-store.de/en/
who sits in Germany and has an active member here in this forum (Booxtor) ships to the US.
For further questions about the different Onyx devices it's best to research our Onyx subforum.