I think you mean the small thumbnails in the file manager, but it would be good if you could specify exactly where you are missing document preview icons, or which ones they are if I am wrong.
Which version of Krita are you using?
Where did you get it from, or what distribution form does it have? AppImage, Flatpak, Snap, RPM, repository, other?
Which Linux distro are you using?
Which window manager are you working with?
What is the name of the file manager you are using?
Some file managers have the option to enable Krita thumbnails, I believe Dolphin is one of them.
Alternatively, you can use the Krita plugin Buli Commander, which functions as a file manager in Krita. Among many other features of interest to Krita, it also offers previews, and even allows you to select the size.
Buli Commander is extensively described here, and can be downloaded from here (because the download link in the describing post is not updated, so not the current release), but you need to patch the current version, which is described here, alternatively, you can download an already fully patched version ready to be imported into Krita from here.
Michelist