Well, I’m a bit late to the table on this, so you may have already heard. But WotC has released (or re-released) some of the older D&D material in PDF format. There’s stuff from B/X, 1E, 2E and even some 3E stuff. They haven’t released all the old material yet, but hopefully they’ll add more over time, especially the 0E books.
The PDFs are available from the following sites: