I've never used the archos to view PDFs. I can try that out later to let you know how well (or not) that it works.
The DVR station is a nice add-on. It comes with a remote and allows you to hook the archos up to a TV. They were too expensive new. I picked mine up on closeout at Walmart I think for about $20. In fact, I think that's one thing that didn't favor the archos; accessories were cool, but all had proprietary connectors/cables, and all were expensive. Also, a major annoyance was that it supported a few extra codecs, but you had to pay for them. Not really a big deal IMO, you can use an app like handbrake to transcode videos from other formats if you want. downscaled transcodes go quick. I'm pretty sure it supports mp4 out of the box, but I can't remember which codecs were built in and which were addons.
I didn't have the wifi version. I've heard it really depletes batteries quickly, but I think you can turn wifi off.
The archos UI is decent, but not super intuitive. It does have a kick ass video preview mode where each vid shows title & metadata, plus a thumbnail sized 3-5 second video loop. It's very cool. I haven't seen this in other portables, but it may be that others do it. On the whole, I liked it, and it was my carry around vid/mp3 player. I'd even say it is quite durable, though uncharacteristically, I took pretty good care of my archos. I still have it, but since getting an iphone, it doesn't see much action. I thought about selling it, but for the low prices they are fetching, I figured to just keep it a as backup.
IMO, the iphone/itouch is superior in most ways, but you aren't going to get a touch for 60 bucks.
The one thing I didn't like about the archos was that it was designed to sync with Windows Media player. I never used WMP to sync. There were a variety of reasons, but mostly because wmp didn't support podcasts. I'm not sure if it does now. I'm not a big fan of WMP anyhow, and I found trying to sync with it a PITA.
EDIT: forgot to say, one really cool feature of the archos 604 is the kickstand. Nice if you have a surface to set it on. :thumbup
Double edit... just checked the PDF browsing. It works about like you'd expect. PDFs render just find. Zooming is a little cumbersome (not like the pinch-spread on the itouch), but since the screen is bigger, I could read it ok. I didn't actually feel the need to zoom in.