hi, I am writing my own website to sell my product which is a pdf
document. I would like customers to complete checkout with paypal and
be returned to a page where they can download the document - or
returned to a thank you fr payment page and navigate to the download
page. However, I don't want the download page to be accessed by anyone
who hasn't paid. Is there a way that a page can be only accessed of
they have come from a certain previous page? or another suitable
solution; for a beginner!

hi budup

I suggest you take a look at the PayPal Website Payments Standard
Checkout Integration Guide @
https://www.paypal.com/en_US/pdf/PP_WebsitePaymentsStandard_IntegrationGuide.pdf