Is it a low-end computer, and do you have a lot of other programs open? Because I'd think that 900+ pages of text would require a lot of RAM, especially if the text is formatted. So maybe your computer has reached its limit? Try freeing up some RAM by closing the other programs (and ideally rebooting the computer), and try again.
Though, I'd think that if it reached its RAM limit then it simply wouldn't copy anything at all (rather than copying 11 pages). And it would probably display some sort of error message. So maybe that's not the problem after all. Plus, this wouldn't explain why the same thing is happening when exporting to *.doc and *.txt, since exporting would be only require disk space, not RAM.
Maybe the word processor is using the wrong encoding and/or maybe there are some corrupt bytes about 11 pages into the document, such that the computer takes these bytes as string-terminators or something, causing it to think that the document is only 11 pages long rather than 900+... both when you try to copy it and when you try to export it, but not when opening it initially. (No idea whether this is possible or not... just a guess...)