If you just want to read then CDisplayEX is good.
For other feature I use Foxit reader.
If you want a light pdf reader then Sumatra is okay.

*checks Sumatra website*
OMG my eyes!

What?
It looks like a perfectly nice 90s website.

Calibre

It's also great for reading ebooks and converting documents from various formats.

I use Foxit Reader because it was forced upon be by GOG and I found it quite good so I stuck with it. :-)

I use Foxit Reader on Windows for a few years now too.

Su Mat Ra!

Foxit is faster for me and looks much better. Reading and manipulation options are also better for large documents (500+pages)

Another vote for Foxit Reader from me.

Lightweight? I just read .pdf in my webbrowser (Firefox).

Another satisfied Sumatra user!
Approves of all things Sumatra :D

Sumatra PDF once more, sold!
or Evince ; p

I'll throw another one onto the Sumatra PDF endorsement heap. It's been serving me well.

It remembers your settings from the last time it was used, so just going back to windowed mode should've fixed it.
I've used Foxit and PDF Xchange as well, but I didn't like the changes Foxit made several years ago.
Calibre is something completely different, although also an amazing free product.
The viewer is a separate executable BTW so you can use that to view docs instead of going into the library portion of the application.