I haven't used some of the newer solutions like DTG and PalmPDF, but I have been using RepliGo (
http://www.cerience.com/products/repligo.htm) for a while now. It's on my list of things to write about, but you know how that goes. So maybe I'll just make a few quick observations here.
Without doubt, RepliGo is an awesome solution, which allows reflowing and various zoom levels and is generally really solid. You have to print the document to RepliGo, so there's a conversion step, but in a way that's a "plus" because you can read ANY document that can be printed, whether it is .pdf or some other kind of file.
The PalmOS version may not be getting a lot of active development, but that's okay because it's so solid, and I can highly recommend it just as it is. And there's a desktop viewer for it also. In fact, once you convert a document, anyone else can read it for free using the free RepliGo viewer for mobile devices, which are available for more than just PalmOS.
I'm really picky about what I keep in my limited Treo memory, but RepliGo is definitely a keeper, and one of my favorite applications!