AppleScript get filename from URL and apply Decode URL
You can use Cocoa’s NSURL class via some AppleScriptObjC:

use AppleScript version "2.4" -- Yosemite (10.10) or later
use framework "Foundation"
use scripting additions

set test to "https://dskjflkosdjfksdajf903094.example.com/g/f/OWZehaoqssVuXRuxqEivP_9dsa89dosidlzkxck_wqezxcd33sda-34sdf34r/%5BEBOOKS%5DEbook%20Title%20-%20Ebook%20Author%20Name.pdf"

set theURL to current application's NSURL's URLWithString:test
set testName to theURL's lastPathComponent as text
display notification testName with title "Added Download"