We are sorry for the trouble. As described you are getting the error 'There was an error opening this document. The file is already in use by another application'
Would you mind sharing the workflow/steps you are doing to open the PDF? Are you trying to open the PDF in a web browser? If yes, please try to download the PDF file to your computer and then try to open it with Adobe Acrobat/Reader DC and check for the issue.
Also, you may try switching to a different web browser and see if that helps.
I will export an AutoCAD file to pdf and instead of opening acrobat and showing the pdf, I get that response. I can save the pdf with a different file name but I really don't want to load my harddrive with multitude of files that way.