Finder not locating PDF files in search, and TXT files vanished in search after opening

Re: "Mac OS Sequoia on a Mac Mini. : Finder not locating PDF files in search, ..." "Finder is not finding words in searchable PDFs for some folders. Finder is finding words in searchable PDFs in other folders."

Wondering if your Mac has the newest available MacOS Sequoia version installed.

(You can check for updates in Settings > General > Software Updates.)

What's new in the updates for macOS Sequoia - Apple Support says macOS Sequoia 15.7.3

and

"This update provides important security fixes and is recommended for all users."

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Re: "I see no actual difference between the ones that show up in Finder search and the ones that don't".

Seems Finder is generally consistent.

Wondering whether you are double-checking precisely where Finder is searching::

If the default search is for "This Mac" you may be sometimes not searching the expected folder, but the Mac.

Eg: When searching in a Finder window:

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Re: "TXT files vanished in search after opening"

Wondering if you have multiiple desktops in use ... eg: One for Pages, one for Safari, etc.

When you search for ,txt files and click to open one .txt file, the Finder search may be left behind on a different desktop than where you are viewing the .txt file content ... it is easy to "lose" the Finder search.

One way can be to begin searching on the desktop you use for Text Edit, or begin the search and bring the Finder window to the same desktop as the .open .txt file using "Mission Control". (It may also be useful when searching multiple files to have text files set to open in tabs instead of in separate files ...)