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Assign keywords to a PDF or image in Preview on Mac
You can assign keywords to images and PDFs opened in Preview, so that you can search using those keywords.
In the Preview app
on your Mac, choose Tools > Show Inspector, then click the Keywords Inspector button 
.Do any of the following:
Add a keyword: Click the Add button
, type a keyword in the keyword field, then press the Return key.Remove a keyword: Select it, then click the Remove button
.Edit a keyword: Double-click to edit it, then press the Return key.
To search for files with specific keywords, go to the Finder, then choose File > Find. Or click the Spotlight icon 
in the top-right of the menu bar.
See alsoSearch with Spotlight