A scanned PDF is useless if the Optical Character Recognition (OCR) misreads Elvish script as gibberish. Verified PDFs usually come from professional scans or official digital releases. Look for searchable text, not just images of pages.
The History of Middle-earth is a scholarly study of the manuscripts from which J.R.R. Tolkien created his legendarium. Unlike The Silmarillion, which presents a cohesive narrative, these volumes show the evolution of the stories from their earliest drafts (dating back to 1916) through to the final published forms.
Volume XII: The Peoples of Middle-earth
Volumes VI through IX form a sub-series that chronicles the writing of The Lord of the Rings. They show how the story evolved from a sequel to The Hobbit into the epic of the Ring.
Volume VI: The Return of the Shadow
Volume VII: The Treason of Isengard
Volume VIII: The War of the Ring
Volume IX: Sauron Defeated