I just purchased the digital version of the Hunt for the Thessalhydra and was wondering if it should have come with pregenerated character sheets I could download, since the physical copy comes with them.  I'm planning on this being my first attempt at DMing and I wanted all the players to use DnDBeyond and the characters the game come with.  I don't want to have to recreate all the sheets in the character builder. 

DDB is working on adding pdf downloads for adventures with premade characters where possible. They've done it with Rrakkma and Locanth Rising, I believe they're working through it where allowed (there may be licensing restrictions with Stranger Things related products)

The character sheets are separate from the adventure book, so the digital adventure book doesn't have a place for them. As for whether there is a legal reason they can't be included, I don't know. I know they have talked about adding the ability to copy pregenerated characters.