Ah, thanks for the link. For some reason that link doesn't turn up in google search for me, however hard I try. Of course the ISO one is also authoritative, but not free.

Oh, mea culpa. Honestly I just found the opening page and didn't notice it needed payment to get to the rest of it. I naively assumed that since Adobe had published it with ISO's permission, it was free in both places.

No worries! How did you find the "opensource.adobe.com" link? I can't hit it with google, even with aggressive searches like "site:opensource.adobe.com filetype:pdf". And I can't seem to navigate there from the main page.

The loc.gov page came up first in my search, and links to the Adobe one :)

Apart from the PDF32000_2008.pdf (the ISO version), Adobe used to have a pdf_reference_1-7.pdf at https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdf_ref... which is the version before it got ISO-ized. The ISO version is "substantially the same" except for typesetting and small differences in wording, but I found the Adobe version much more of a pleasure to read. Comparing them is a good exercise in how much these small differences matter.

kinda funny you need a pdf reader to read the pdf specification :)

Yeah, how did they bootstrap the first pdf reader? /s

It must have been hard guessing at the spec until you could read it properly.

Maybe they used Postscript?

I somehow thought early versions of the PDF spec were published as a .ps version for that very reason, but my duck-fu is failing me finding any such link. It may require wayback-fu and that's beyond my level-of-effort :-)

Its same with html spec I think.

HTML is somewhat human readable in a text editor, but PDF likely is not.

Actually a lot of the PDF format is plain text, but can contain binary streams. You can open a PDF in a text editor and see the header, and skip to the end and see the xref index and some other parts. The binary sections are enclosed in plain text start and end markers, but you probably won't be able to read much of the actual content this way since it will be compressed or encrypted.

Sometimes even the binary fragments are compressed plain text.

How did you find the link on opensource.adobe.com? The used to host other standards, too (e.g. font formats).