Before the great purge, when WotC removed their PDFs from the OneBookShelf / RPGNow sales platform, one of my purchases was the Original Dungeons & Dragons OCE Boxed Set in PDF. It was handy, even if it wasn't the cleanest copy. After the purge I wan't even able to redownload a copy if I wanted to.
OD&D is back at RPGNow. I have access to the files again and it's very clean, but that makes senses, as they are using the reprint files for their source materials.
$9.99 get's you the contents of the Original Dungeons & Dragons boxed set, as presented in the reprint WotC did 2 years ago. The booklets have the new covers, not the originals (sadly) but the contents are there.
If you are looking for an inexpensive copy of the Original D&D rules, this is an excellent way to get your hands on them.