This app was mentioned in 13 comments, with an average of 1.77 upvotes
Try this - https://play.google.com/store/apps/details?id=com.httrack.android&hl=en, I didn't try it myself but I think it does support PDFs.
btw I'm the developer behind Offline and I'll eventually add support for PDFs (although it's really low on my todo list).
If you can only find online sites, there is still a way to get it offline. There are apps that download entire websites to your storage, and then you can use a browser to open the offline file and you can just navigate the site like usual.
https://play.google.com/store/apps/details?id=com.httrack.android
Keep in mind, however, that a large database like the one you are looking for will take up quite some space on your phone, and downloading it will take some time.
If you are not averse to using the terminal, you can use curl in Termux
Or try HTTrack Website Copier
If you try to go to Facebook.com on a browser but have no internet connection, nothing will load. Because it doesn't have access to Facebook.com, or any of the resources that page needs to load. Using Android? Go download HTTrack website copier. https://play.google.com/store/apps/details?id=com.httrack.android
HTTrack does the job for me.
So long as your Chromebook can run Android apps..
How about this one? HTTrack
https://play.google.com/store/apps/details?id=com.httrack.android&hl=en
What about HTTrack Website Copier
OwO what's this
WinHTTrack for Windows
Android:
https://play.google.com/store/apps/details?id=com.httrack.android&hl=en