Hello, perhaps I want to order a Kindle Scribe. But I've one question - can I upload books to the Kindle with an USB cable connection (with Calibre)?
You can, but like all Kindles, Series and Collections and covers is an issue.
You can also copy PDFs, but unlike virtually every other eInk (or Tablet)with a digitiser stylus, you can't annotate them. Annotation is only possible by sending the PDFs to Amazon.

It's large for ebooks, the size better suits simply reading PDFs (I have two 10" approx ink and 10 " tablet as well as smaller eink from Amazon and Kobo).

Unlike Elipsa there is no handwriting conversion. It's only worthwhile compared to alternatives for PDF reading only as stylus based annotation is a clumsy workflow with no privacy.
Thank you, Quoth - I will mostly use the kindle to read PDF files (instead of reading them on a tablet).
You can annotate side loaded PDFs with highlights and notes, as always, just not with Sticky Notes (if they have text layer to highlight and attach notes to, of course).

But the weird thing is that the Kindle apps don't allow any annotation at all (apart from Bookmarks), even when they are sent to Amazon. They would do well to support that with a future update. If they want Kindle to be a note taking platform (Scribe is not a platform), it doesn't help their case by having gaps like this.

Amazon bought a stake in Foxit some years ago, I think that technology is what they're still using for PDF on Kindles, but don't know how to verify that.
I see a lot of users saying they can't annotate/use sticky notes on sideloaded PDFs unless you send it through Send to Kindle.

Is it definitely the same for sideloaded ebooks? I did try it myself (sideloading epub and azw3 to the Scribe via USB cable) and found sticky notes don't work, but since I couldn't find anyone talking about it I wondered if I'm just doing something wrong.

I really hope future software updates address these issues, but I have a feeling it was engineered that way on purpose.
From everything I've read, it doesn't work sideloaded. You can do the email thing, which causes Amazon to do its behind-the-scenes conversion touch, then sticky notes will work
Another experiment would be to try Download & Transfer via USB, to see if that works for Kindle books and allows you to create Sticky Notes.
(edited: reply in wrong thread and don't know how to delete)
Have you tried KFX? If that works?
I've been testing and you can use hand-written sticky notes just with kfx format, not with azw3. In fact, floating note tool is disabled for these formats. So, if you use send to kindle option, the ebook will be converted to kfx and you'll be abble to write sticky notes by hand. But, note that azw3 or mobi are not allowed formats to use with "send to kindle" option. You can use epub format, for example. If I'm wrong, please, someone correct me.