If you revisit PDFs opened during the past week, save that Finder search as a Smart Folder. Input: Kind is PDF and Last Opened is within the past week. Result: a live list in the files’ original locations that updates as matches change. Apple’s Smart Folder guide

Save the search

In Finder, choose File > New Smart Folder. Add Kind = PDF and the recent Last Opened condition, set the scope to the current folder or this Mac, then save and add it to the sidebar. A Smart Folder saves criteria; it does not move or copy files.

If the result is empty or noisy

Check the scope and whether the external drive is connected. If no files appear, temporarily remove the date condition and see whether the PDF rule matches; if too many appear, add one rule at a time to see how the result changes. Use “any of these” when one of several terms can match, and all-rules logic when every condition must match. Macworld’s dated example narrows a search to documents worth finding again before tagging them. Macworld example

Open a result’s original location if you need to copy the actual file. Use tags when you want to retrieve files by project or status without moving them.

Related: Group files across projects with Finder tags

Sources