If user tries to print an invoice using a custom and/or standard invoice form and receive the notice "The main form and remittance form cannot fit on a single page. To make them fit, you can edit your forms and/or layouts", check the following:
1. Navigate to Setup > Company > Printing & Fax
2. Go to Printing tab > Transactions section.
3. Set Print Transaction Forms Landscape = False (unchecked).