The reason is because the current notepad invoice everybody could open for the last 20 years is now reformated by various smartphones and devices the customers use … thinking JPEG is common and won’t be reformatted like my old notepad invoices yet can be opened easily
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The PDF format is usually used for archiving.
Open your spreadsheet document in Calc.
Choose File >Export as PDF... from the menu or click on the Export directly as PDF icon.