I have to agree with Elsi--a netbook may be what you're looking for. You can pick one of those up for $350 or less.
The Sony 700 (which may have been discontinued and is getting harder to find) is better for viewing technical PDFs than the 505. Although the screen contrast suffers some in comparison with the 505, it does a much better job IMO of viewing PDFs. (My husband has a 700 and I have a 505.) The 700 has real zoom and search capabilities, and it also has a built in light, so you don't need the wedge.
There are rumors that Sony will be releasing a couple different units, so you may want to hold off to see what's coming down the pipeline.
Good luck!
dordale