IMP-22: Group PDF invoices in one

[albertosml]

Desired flow steps:

• Go to invoices (customer or trader).
• Select a date interval to get the invoices PDF.
• For each selected invoice, get the PDF.
• Combine all those PDFs in one.

[albertosml]

Alternatives to solve this problem:

1. Use this program.
2. Create an output spreadsheet where I will add copies of each sheet of each selected invoice. Finally, I export that global spreadsheet to PDF, so we have all invoices combined on a single PDF.