Deleting a document from a job does not delete the attached file altogether, rather removes it from the job. The file will still remain in the folder for that particular job, if one exists.
From the Setup menu, select Jobs.
Select the specific job using the Jobs Toolbar.
Click the Document Attachments tab.
Click on the file name to remove. To delete multiple files, press the Shift key and click on multiple file names.
Click the Un-Attach button.
At the confirmation, click Yes to remove the file(s). Or, click No to cancel.
Click 
to save the changes.
Overview of Job Document Attachments