The game includes a manual and 3 wallpapers as extra goodies.Nothing is mentioned on the store page about a pdf walkthrough.

If its not listed in the goodies section on GOG, ask the developer. They might have forgotten to include it.

note in my first post, Cyan said it was included

https://obduction.fandom.com/wiki/Obduction_Exploration_Guide

I did some research and the best I found was that it was a reward for people who originally backed the game and was not made available to non-backers.

Have you checked the install folder to see if it's there? GoG sometimes just bundle with the offline installer rather than having as a separate download.

Otherwise, ask the developer to check that they've put it in the GoG distribution of the files.

You need to contact the developer if they are missing, gOg can not publish what they do not have.

May I just point out that the wiki do not say it is a 'goodie' here on gOg? gOg is not mentioned anywhere on that page. What it says is "The original document is found in the Obduction game files as a PDF file." - so have you checked in the game folder? it may be among documentations there.

It may also be that the wiki refers to different versions of the game, for example the physical copy, which may be different from the digital copies.

There-in lies the probable answer. I've noticed that at least two sources have a paperback version, most likely a copy of the original pdf