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I need to modify the sales invoice print-out format from existing one to a simpler look.
Do you know the easiest way to do this by ourselves?
You can easily modify the existing template by using the Report Designer from NAV Development Environment.
unless you use Word layout for your invoice (in which case it is relatively simple to change), changing the RDLC layout requires good development skills and report designer (likes of Visual Studio). If you don't have experience in creating/changing reports in Visual Studio, I would recommend finding a developer who has, otherwise there is a big risk of messing up the layout.