You are doing this very inefficiently. File::Find goes to the trouble to chdir into the directory that holds the files you are looking at and then you do operations on a long path string¹ which means the operating system needs to reparse the path and pull information about all of the parent directories out of (and perhaps in to) the cache.

First, set $File::Find::dont_use_nlink= 1; (to make your script portable -- it won't be using this on Win32 since Win32 file systems just don't work that way), then use -M _ instead of your current -M $file, unlink($_) instead of unlink($file), and (much less importantly) use $_ =~ m|\.pdf$| (and you can drop the $_ =~ part if you so desire).

Note that I said -M _ and not -M $_, the former being a bit more efficient because $File::Find::dont_use_nlink= 1; assures that File::Find has already done an lstat on $_ for you so you don't need to do it again.

¹ ...which might even be incorrect if $ARGV[0] didn't contain an absolute path name. I'd have to reread the File::Find docs to determine that, but what you do to fix this is the same regardless so I won't.

(updated: trivial)