While reviewing some PDF articles I took notes that I wanted to delete later on using macOS Preview app. But it didn't work as expected.

Here's the crime scene:

The most obvious things to try are hitting the backspace key (also together with ⇧ and/or ⌘). But this didn't work.

Lucky me, there's a workaround.

Open the "Inspector" using ⌘ + i shortcut:

There you can select a note and once it's highlighted, it can be removed using the backspace key: