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He typed (edit: and handwrote) hundreds of these open letters, technical notes, reports, etc. Here's the archive at UT:

https://www.cs.utexas.edu/users/EWD/

> IN 1959, DIJKSTRA began writing a series of private reports. Consecutively numbered and with his initials as a prefix, they became known as EWDs.

https://www.cs.utexas.edu/users/EWD/welcome.html

That's the archive, talked about in the linked article. Letters (mostly) that he wrote and sent out to people. Today we'd consider their content and length equivalent to a technical blog. Some were much longer, though.