Updated to Big Sur and find -exec results in permission denied
The problem is with your find invocation:

find . -name &quot;*_20*.pdf&quot; -exec {} ~/Desktop/. \;

If your PDF was named “Foo_20.pdf” then when find matched it it would attempt to run Foo_20.pdf ~/Desktop/. which is an attempt to execute the PDF as if it were an executable, and of course because it’s not (specifically, because it lacks the +x file permission) you get a permission denied error.
Your command is missing a cp invocation:
find . -name &quot;*_20*.pdf&quot; -exec cp {} ~/Desktop/. \;