I have configured my CAJA to show preview for "local files only", so I can see previews of many kind of files, but for PDF files I always see the Adobe logo, on the desktop or inside a folder.
What should I do to see previews of PDF files?
On my system it worked out of the box for PDF
That said; preview is skipped if the filesize exceeds a certain threshold.
You can change that threshold here:
Also, check of 'Atril Properties' is selected in the tab 'Extensions'
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@tkn thanks for your reply, well in my CAJA properties the file size is set to 100 MB and the "Atril Properties" are selected. Both was like that since the beginning.
Do you have the PDF-reader 'Atril' installed (and set as default pdf reader) ?
If you have, then I'm all out of options at the moment. sorry. 
Yes, when I double click a PDF file it automatically opens Atril.
That's weird.
Oh , before I give up totally. Try this:
sudo apt install --reinstall caja-extensions-common
It could be that it is damaged or partly installed (or not installed at all), and you likely need it.
Sorry, was not the solution. I reinstalled the package and restated the laptop, but I still see the Adobe logo.
I manually cleaned the cache (~/.cache), no results.
I tried to change the default PDF reader, no results.
I removed (purge) and installed again Atril, no results.
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Have you tried deleting the thumbnail cache ?
rm -r $HOME/.thumbnails/*
(afterwards logout and login again)
Actually the command is rm -r $HOME/cache/.thumbnails/* but doesn't do the magic.
Actually the command is rm -r $HOME/cache/.thumbnails/* but doesn't do the magic.
Accordig to freedesktop.org that is correct, or to be precise: that should be correct.
As a matter of fact, there are two places where thumbnails are stored (at least on my systems):
$HOME/cache/.thumbnails/
and
$HOME/.thumbnails
No more ideas to fix this problem?
Hi, @smurf (Luca DG),
Besides the good suggestions that @tkn (thom) has already given you here in this topic, I've found the following similar Topic / Conversation ("Can't get PDF Thumbnails") here in the "Ubuntu MATE Community", started by @koana33 (Dakinu) about 2 years ago (on June 2020):
Reading that topic, it seems that one other thing you might still check is to ensure that the "Document Viewer" in the "Office" tab in "Preferred Applications" is set to "Atril Document Viewer" as @olek (Alexandre) shows at the end of the following message in that other Topic:
I hope this helps in some way.
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Hi ricmarques,
thx for your reply.
Yes it was marked with Atril, I tried to change it and reboot, but nothing changed.
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Just to let you know.
In the last week I didn't do nothing more to try to fix the problem. This morning I booted the laptop and magically I see that the pdf files on the desktop have the preview in the icons, but the files in folders and sub folders don't.
Weird.
speaking of reboot... have you tried completely shutting down (not a reboot)... I have seen that fix things and I think a reboot is different than a complete shutdown/restart. Worth a shot.
Yes, oif course, every single day in the night I completely shut down my laptotp.
Anyway is surprising that if I move a PDF files from the desktop to a folder the preview disappears and the adobe logo appears.
That is very strange indeed 
Now you have me thinking. I will see what my Ubuntu MATE 22.04 system does with PDF thumbnails
Hello
Not an answer but in my UM22.04.1 the preview seems to work ok. I had to enable as prefer to have them off. Also in my folders with pdfs they show previews
Top is pdf on Desktop and open folder on right
Bottom is Desktop and pdf now in open folder
I attach a short video to let you see what happens on my system.
https://drive.google.com/file/d/1dT7JhffctQFzVeVJxhvHsvDeRH-4NnMW/view?usp=sharing