Because you would have all of your document's pages loaded into memory. That may not be the ideal situation depending on your machine's available memory. Not a big deal really, just something to consider.
In that case you probably want to look into some sort of caching arrangement, where your pages are swapped to disk while you run through the document. Something like Cache would probably do the job nicely.
Once you get to the end of the document, you can go back to the first page, add the "1 of 3456 pages" to the page, write that page out, flush that page from the cache, and continue to the end.