When attempting to move a PDF file from the iMac desktop to an external drive, receive the following Error messages: 1) "The Finder can't complete the operation because some data in "...... .pdf" can't be read or written. (Error code -36)" -AND- 2) "Adobe Acrobat could not open '...... .pdf' because it is either not a supported file type or because the file has been damaged (for example, it was sent as an email attachment and wasn't correctly decoded). To create an Adobe PDF document, go to the source application. Then choose Save as Adobe PDF from the PDF dropdown in the Print dialog." --- The files do open and can be edited on the desktop without any problem; but, it will not allow copying or transferring to an external drive. WHY? This just started to happen within the last week; before then never a problem moving to backup drives. Would appreciate any help available. Thank you!