Yes, parish books of Porstendorf (Borsov now) are digitized, but you cannot view it online, you can only download whole book (free of charge).
Go to: http://www.archivzamrsk.cz/
Choose English language
Click on “genealogy" link
Click on “parish registers in Zamrsk“ link
Click on “inventarni seznam/inventory register“ link
Fill in “borsov“ into the right field on top bar. The page for Borsov will appear. There will be 21 parish books listed on following pages (inventory numbers 359 – 379). Matrika = parish book, Index = name register, N = births, O = marriages, Z = deaths.
Click on the last row end (ending “zip“) of chosen book and download it.
Good luck in research.