I've been looking at Archive.org, and there are several abridged editions of the book. From what I can gather, there is an abridged version that is technically out-of-copyright. It's published by Barnes and Noble Classics as well as Signet classics (The B&N book says that "the present translation and abridgement [...] remains anonymous" and the copyrights are only for the introduction, written by another person).

The tip off is the number of chapters (71) and how "The Departure" is Chapter 70, and "The 5th of October" is Chapter 71, so it's pretty painful and obvious about what got cut out. I tried to read it, and it seems to me that the language was not updated to contemporary English, so even the abridgement still has somewhat clunky and outdated language. So shorter is not necessarily better, or easier for a younger reader to comprehend, especially when making the book shorter just appears to mean "hacking out chapters", as opposed to a complete rewrite and more careful excising.

On the other hand, the 1956 translation and abridgement by Lowell Bair is a delight to read. Very understandable, and preserves a lot of the delightful dialogue, and the full consequences of the Counts revenge on Caderousse, Fernand, Villefort and Danglars are true to the original text. It even includes mazolatto and Eugenie and her girlfriend sleeping in the same bed! (gasp!) That version flows beautifully, and is good for lighter reading. All in only 531 pages.