Another vote for Okular.
KDE default, and it’s always worked fantastic for me in the past. Actually, lol, I was just giving a presentation a couple weeks ago using it, and – I am not making this up – one of the people in the audience was so impressed he came up to ask me what program I was using.
That’s just because ZFS’s disc cache shows up as application-used memory, instead of disc cache memory. (Disc cache memory is often reported in system monitor tools as ‘free’ memory, because it can be freed up as soon as you need it. ZFS’s disc cache works the same way, but it’s not reported separately to the OS, so that makes it look like ZFS is using a shitload of RAM.) If it bothers you, you can set a maximum amount of RAM for ZFS to use, and that will clear it up. It will mean that your performance is slightly worse, though, because ZFS won’t be able to use disc caching optimally.
Not very ‘lightweight’ if all you need it to do is read PDFs.