Based on my test, it seems possible to use a relative path.
I uploaded an excel file and a pdf file named 01.pdf to a same folder in a team site.
In the excel I used the format 01.pdf as the relative path in the hyperlink.
Then I tried:
- moved the files to a subfolder in the same site
- move files to another team site
- download files to local disk
the hyperlink all worked for me.
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