Depending on how many items you've added to an invoice, it can extend onto a second page. This can be changed in the browser print settings. For example, when printing in Chrome, you are able to reduce the scale of the document to fit on a single page. Click on More Settings and reduce the scale until the invoice fit on a single page (e.g. 90%). You can also select "print on both sides" to print on a single page.
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