I was looking through my digital copy of Starship Titanic and I noticed that what's labeled as "manual" is actually the In-Flight Magazine. While a nice addition, the actual manual of the game is missing.

As such, I've attached a PDF I scanned of my own copy of this manual, and hope it'll be a sufficient addition.

That's good of you, but I'm not seeing any attachment. If I remember correctly, the GOG Forums blocks user attachments until you've built up some reputation points. This is to help mitigate spam using autogenerated accounts.

What else can I do, then? I still have the individual page scans as well.

I asked all this because I had to re-check my physical manual to know how to control the final puzzle - which I understand is skippable.

I'd suggest uploading your scan somewhere and provide a link in the message. Actual links may be prevented at your rep level, but if you just paste it as text in a message, it should be fine. I'd suggest maybe the replacementdocs website as a good place to have it hosted permanently.

I actually had the PDF and source scans uploaded on a cloud drive, here: https://drive.google.com/drive/folders/1ZWnBmX0ie4jO0Rfb3x_FPCzJzm7eh07A?usp=sharing

If this is still a risk, let me know.

I had no idea we were missing the actual manual here! Currently, your link isn't working, so might have to reupload, maybe.

If you or anyone else have a copy of it, would appreciate!

try this one. This marvelous russian site has pretty much all, also the manuals. trustworthy as far as I can tell.
https://www.old-games.ru/game/download/get.php?fileid=25705&modal=1