How to set Thank-You Letter template to start the next letter on a new page
I cannot tell you how to do a page break and it probably also depends on your pdf library (e.g. dompdf or wkhtmltopdf).

You could however consider using one of the existing extensions for donation receipts to create the Year-end receipts. There are several around, one we created quite a while ago can be found here: https://github.com/systopia/de.systopia.donrec

The documentation can be found here: https://wiki.civicrm.org/confluence/display/CRM/DonationReceipts+Extension