KOReader

Short version: there is no reliable way to run KOReader on a Kindle Scribe right now, and even if there were, it would turn off the features you bought the Scribe for.

Why it is not practical

• A jailbreak is required, and Scribe support lags. Like any Kindle, the Scribe would have to be jailbroken first. Jailbreaks are tied to a specific firmware version and are released later for the Scribe than for the Paperwhite, when they are released at all. For long stretches there is no current method.
• Firmware updates would undo it. The Scribe updates itself often — its note-taking features are still being added to — so any jailbreak would be fragile unless you blocked those updates and stopped getting the new features.
• KOReader has no pen support. It is a reading app. It does not do handwriting, notebooks, in-book notes or PDF markup. On the Scribe you would lose all of that the moment you opened it.

What to do instead

• For typography and PDF reflow, the Scribe’s own reader has improved and handles large-page PDFs well. For dense PDFs, zoom and pan, or read them on a computer.
• For your own EPUB books, use Send to Kindle or Calibre.
• If a maintained Scribe jailbreak appears for your firmware, the MobileRead forums are where it will be documented. This page will be updated if the situation changes.