Hello. What should I do to make Dolphin "search" function work? I guess it's a problem with accessing files in partitions separate from "home". In the settings, all I could find is the possibility to exclude locations (partitions) from search, not how to enable it in these locations.
Thanks.

If you choose "Everything else" it's limited to home, navigate to / (root dir) and perform search for all disk.

Sorry, I don't understand your recommendation. Let's talk about a specific user case: I want to find a specific pdf file in a large folder on a specific partition (not "home"). If I simply apply your method, I cannot find anything - and it doesn't make sense, because the "search" should be directed to the specific folder where I operate the search.

It depends how you mounted the partition. Searching in dolphin is done from the folder you are located in, fx. ~/Documents. If you want to search from within another partition you have to open that folder in Dolphin, fx. /mnt/Storage. Search everything is same as searching from your home folder, and searching from root performs a search for everything included mounted partitions.

Thank you both for the answers. I discovered a way to make search work in dolphin: clicking on "Find" opens a few tabs with some settings for the search options, and then I select "Everywhere" instead of "From here (user)" , as bvbfan suggested. I guess that in another KDE distros the default setting is Everywhere, and this is why I was confused about not getting the results I wanted.
All is clear, now.