Invoicing software

To make customized Invoices to bill customers for incoming calls through our PBX and Credit Notes to pay telephone agents for time worked. We make and supply interchangeable background images for the LibreOffice Calc Spreadsheets. We put 3 small spreadsheets on the same page but the client only sees the background image ! Before being uploaded to the server, the spreadsheets with background images, including all calculations , must be changed to pdf with open source software (ex; AstPP - to be discussed), without loosing anything, Than they can be filled out on the Internet by our clients, changed to pdf again or to png and send by email, as invoices or Credit Notes, to their final destination. The client must never see the spreadsheet, he only sees the background image with the data he entered into the Cells ! Min. knowledge: 1) LibreOffice Calc Spreadsheets. 2) Software (open source) to change the spreadsheet files to pdf, without loosing calculations ! 3) Asterisk PBX would help This is a none standard project and absolutely not for beginners !! Very good knowledge of the way Spreadsheets and Billing software work, is needed. We have a Multi Disk Server with KVM disk management and MySQL DB ready to upload. -- Good and quick communications are essential ! You must have a computer with a Linux OS --- This is not for Windows !! Please read and understand the file attached to this project, for step by step instructions ! Project will be divided into 2 milestones.