Currently Being ModeratedDec 4, 2012 8:37 AM (in response to Gary Goldblum)
Set in the Finder.
For individual files, select the file in the Finder, then press command-I to Get Info.
Click the disclosure triangle to open the Ownership and Permissions section of the Info window.
Change all three pop-up menus in the Details section to Read Only (you may have to unlock, using your password).
Currently Being ModeratedDec 4, 2012 10:47 PM (in response to Gary Goldblum)
Just have a template of the invoice, which always open as untitled and will not be overwritten.
Keep the folder of your invoices open to the side in Finder so that you can manually update the Invoice number which you have to do anyway.