Sure. They are just text.
The difficulty is only in the amount of memory available. With only 2K of RAM you have to keep the text stored somewhere else. Some people keep it in PROGMEM, others use a SD Card. The challenge with SD Card is it is cumbersome to transfer segments of the file (much less than 2K) from the SD Card to RAM to the Ethernet controller.
If you need your Arduino to serve up extensive web pages, maybe you need to look at a microprocessor oriented board.