When you save a document in Business Manager , changes made to the document file outside will not be visible.
Prepare a file called 'TestInvoice.doc' and save it on your desktop.
With Business Manager , Click 'New -> Document' menu. Fill mandatory fields and choose 'TestInvoice.doc' residing on the desktop as document file. Save the Document.
Close Business Manager.
Make changes to 'TestInvoice.doc' residing on your desktop and save it. Open Business Manager. List/ Locate Document that you saved in step 2. Right click and click on 'Save Document -> ...'. Save it under C:\temp. Open it and compare it with 'TestInvoice.doc' on your desktop.