KDE: how to enable PDF preview in Gwenview? [SOLVED]

[Hamlet]

I would like a program to show thumbnails of all PDF files in a directory. My environment of choice is KDE, so Gwenview is a natural candidate for that.
But in its directory view Gwenview does not even list the PDF files. In the Internet I have seen people asking how to disable the preview, so I suppose that's an option.
What do I need to configure/install to enable that feature?

I am running Gentoo/Linux, weekly updated, and I have merged kde-plasma/plasma-meta plus single KDE packages (no other KDE metapackage).

[fedeliallalinea]

I think progra to create thumb in dolphin is kde-apps/thumbnailers.
For active thumb for specific file type in dolphin go in Settings -> Configure Dolphin... select General in side bar and switch to Previews tab.
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[Hamlet]

My problem with dolphin is that I am only able to see one preview at a time. I would rather have a gallery of many files, to be able to spot the one I need.

[Section_8]

[Hamlet]

Thank you.
I had a lesser hiccup in that the menu is not shown where I expected it: in my layout (which came sort-of-default), Dolphin has a hamburger menu on the top right, whose last entry ("More...") holds the full "traditional" menu.
Also, I could not find a "View mode" in the menu, but the equivalent buttons are shown on top left (default keyboard control for icon view is <Ctrl>+<1>).
The icons are very small, but it is possible to zoom in a bit (<Ctrl>+<mouse wheel> or a slider on the bottom of the window).
So in the end I mark this solved, although I ended up using Dolphin rather than Gwenview.

Thanks for your help!

[Section_8]

If you prefer the "traditional" menu in dolphin, I think CTRL-M replaces that hamburger with a menu bar across the top.