hi Dirk, unfortunately if you're running reports locally then you can't find the title using the method that Sohel suggested.
if you open up the pdf in notepad the report name is not there either unfortunately so the only thing that you could hope for is if the person that did the export didn't specify the name of the pdf and just exported directly to a file keeping the report name as a default.
only one other suggestion is that if the report was saved with data at roughly the time that the report was exported, then the file date will be different. so a search on all files using Date Modified in windows file explorer may be an option.
cheers,
jamie