nopCommerce 3.50- 3.70 How do I modify the invoice?

Modifying the invoice in nopCommerce has two parts.
1.  Items that are configurable.
2.  Changing code.

Items that are configurable in the standard Invoice for nopCommerce 3.50 are...
1.  The logo
2.  The footer
3.  Order notes

Items that must be configured in code include...
1.  The layout
2.  Adding text blocks such as a "Remittance" block

If you need to modify the code...
1.  Download the source code.
2.  Edit Libraries\Nop.Services\Common\PdfService.cs...look for PrintOrdersToPdf function.
3.  Build new Nop.Services.dll (found in webroot\bin)...copy to website (webroot\bin).

nopCommerce 3.50 uses the open source iTextSharp libraries for printing to PDF.  The iTextSharp people will answer questions at StackOverflow.