DC Comics wasn't the first digest comic publisher by any means--Harvey Comics had published them as far back as the 1950s. And their first effort was as random and practically anonymous as it was for their treasuries--a single issue based on a non-DC-Universe property. For the treasuries, it was Rudolph, and for the digests, it was Tarzan (bottom of page). DC put out Tarzan Digest #1, reprinting some of the best of the Joe Kubert Tarzan comics(and over a whopping 150 pages!). But they wouldn't publish another one for another seven years.
Then, in late 1979, as the treasury format was winding down, DC jumped into the digest format hip-deep. They created two titles, Best of DC Blue Ribbon Digest, and a year later DC Special Blue Ribbon Digest. Like the treasuries, they consisted almost entirely of theme-centric reprints. Once in a while, they would burn off some inventory material in them (like in Best of DC #4 with its all-new Rudolph material (originally scheduled to be a treasury!), and Best of DC #41 featuring all-new Sugar and Spike stories. In most cases, the "all-new" material was inventory, not new work created especially for the digests.
By the time DC was doing digests, Superman and to a lesser extent Batman were the name(s) of the game in comics--in the first 20 Best Ofs alone, Superman was the star of ten of them, and at least appeared in four more! Supes for the most part would continue to dominate the digests, as this was the time of the first two Superman films. With the DC Special book, they were a bit more adventuresome, devoting issues to such lesser-known titles and characters like Justice Society, Sgt.Rock, Ghosts, The Warlord, the Haunted Tank, and Dark Mansions of Forbidden Love(!). Interestingly, the only character seemingly popular enough to get his own digest was...Jonah Hex?? Yep, Jonah got three issues of his own digest, called Jonah Hex and Other Western Tales. It debuted the same month as Best of DC #1, and featured beautiful painted covers (see bottom of page).
DC also devoted some issues of their seemingly all-purpose title DC Special Series to be digests (they also published a treasury or two under this banner)--Sgt.Rock, Secret Origins, World's Finest, and Flash and His Friends (sounds like a Saturday morning show). Why DC felt the need to do these separately is anyone's guess. The digest format must have seemed pretty viable for a while, since when DC brought back their (at the time) longest-running book, Adventure Comics, it came back as a digest! While it did feature some all-new material, it was mostly reprints. But to DC's credit it was some their finest material--like the Skeates/Aparo Aquaman, a 2-part Alex Toth (!) Black Canary story, some Neal Adams' Spectre issues, and some other fun stuff. Unfortunately, this new version of Adventure must not have sold so well, since after only 12 issues, Adventure was cancelled, never to return.
Best of DC Digest lasted by far the longest, lasting all the way into 1986. They did frequent "Best of the Year" issues, which by the mid-80s were almost always half-filled with Alan Moore stories. Steve Bissette and John Totleben's ultra-detailed art on Swamp Thing was especially poor-suited to being shrunk to such a small size (and sometimes DC made it even worse with their glitch-filled "Flexographic" printing process--yeesh!), and as comic art got generally more detailed to reflect better printing techniques, the digest format seemed a little outdated. After one last "Best of " issue, DC ended their run of digest comics with #71. Not to overthink it too much, but consider the cover subject of the final issue--Ambush Bug. What better character to represent the kind of tonal change comics was going through at the time? Young comics fans were almost all gone, and the ones that were left (like me), thought themselves more sophisticated readers, and wouldn't be satisfied with thirty-year-old reprints anymore.
Digests have sort of made a return to DC in the last year or two with their reprinting of several of their Cartoon Network titles, like Justice League, Scooby-Doo, and Superman Adventures. While I find DC's way of marketing these books...peculiar, to say the least, I'm glad to see them back. Since the CN books generally use cleaner, less detailed art, and with the better paper and even better printing, they are ideal for the digest size (even if they are a tad bigger in dimensions than the "classic" digests). DC keeps putting out more, so I assume they sell fairly well--I'd love to see a full comeback, with maybe DC reprinting some more of the metric tons of classic material in the vaults in the digest format...