Is PCManFM in Solus repositories, or I need to compile it from source?
I have tried with $ eopkg install pcmanfm, but I assume, since I'm using KDE, package might have a different name, pcmanfm-qt or something like that.

The easiest way for you verify this is to open Solus Software Center and search for it.
As far as I can tell it is not in the repo.

sudo eopkg sr cman
should find everything with the string 'cman' and also your package if it is in the repo.

As Sebastian said, neither pcmanfm-qt, nor the lxqt desktop, are in the repo. I would love to see a Solus lxqt version, but I don't think lxqt and xfce would be maintained at the same time.

Speaking of Xfce: we have thunar in the repo. And Nautilus, nemo & dolphin.

Thank you for replies guys, yeah definitely not there. I will compile it then.