The file uploader control is very generic and usable for all kinds of files. I would like to keep it that way.
I don't think it is possible too, your request would require additional PDF reader libraries (java) to read the count of pages. These libraries can only be used and called from server side. The file uploader uploads the files from the browser (client side, using javascript, not SSJS), the limit check is peformed by the browser itself.
So the only way to accomplish this is by checking for all the pages AFTER the files are uploaded and saved on the server. But this goes beyond the scope of the file uploader. I think you should create your own custom validator for this.