I try to find the correct format specification for JPEG/JFIF/EXIT, etc.

I first check this post, but Chrome prevent me to download all the PDF files by showing below:

Implying the files contain viruses.

I then have to Google and find some more, as below:

1. https://jpeg.org/jpeg/index.html
2. https://www.w3.org/Graphics/JPEG/jfif3.pdf
3. https://www.ecma-international.org/wp-content/uploads/ECMA_TR-98_1st_edition_june_2009.pdf

So I totally get confused. The domain of these sites looks very official, they are all reputable organizations. Then which one is the official format specification? And are they the complete version that covers all JPEG variants?