Assign keywords to a PDF or image
You can assign keywords to images and PDFs opened in Preview, so that you can search using those keywords.
Choose Tools > Show Inspector, then click the Keywords Inspector button .
Do any of the following:
Add a keyword: Click the Add button .
Remove a keyword: Select it, then click the Remove button .
Edit a keyword: Double-click it.
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.