I created an excel spreadsheet with hyperlinks to open pdfs in a folder. I am getting "cannot open file" error.

Has it ever worked? Is this the first time you've tried this?

Try it with a single cell first... no sense creating a slue of cells with hyperlinks that won't work.
In a cell, type in your reference text
Highlight the same cell again and press CTRL-K
Find and select your PDF reference file, then click OK
Immediately try clicking the new hyperlink to make sure it will open the external file
If PDFs don't work at all, try another filetype to see if ANY hyperlinks will open for you
Try a .jpg
Try a .doc file
Try another .xls file
If some filetypes will open but NOT pdf, then try reinstalling your PDF display program.