There was no such thing as a PDF document in 1961.
Therefore, this is not the original document.
Since it is not the original document, it stands to reason it is a manufactured document.
So just show us the original documents.
You’re absolutely correct. This particular computer file is not an original document.
The more-original document is on paper.
A computer file is produced by scanning the paper document. In this case, it was scanned, then optimized using computer software and converted to a PDF file, which was then posted to the web.
Actually, in this case, the ORIGINAL document almost certainly no longer exists.
The original document would’ve been done on paper, and entered into a physical book. Over the years, the original document was scanned (most likely onto microfiche). I don’t know for sure, but I strongly suspect that the microfiche records were subsequently digitized, and all of the “original” information is now stored in digital form, in an image database.
From there, an image would be printed onto safety paper, stamped and sealed to create a certified copy of a birth certificate.
Reportedly, what we have is a scanned, optimized, converted-to-PDF file of one of those green-safety-paper, certified documents.