Is it worth hiring a travel guide to see Portugal in 5 days, or is it easy enough to explore on your own?
Portugal is a small country and you do not need a guide to explore it. But in five days you will not see the whole country. You have not stated what part of the country you want to see. You could easily spend all 5 days in Lisbon, doing a couple of day trips to Sintra and Cascais. Or if you want to explore the north you can stay in Porto. Or if you want the south you can stay in a town in the Algarve. Get the RS Portugal guide and narrow your area. Then pick what you want to see. But with 5 days we would recommend picking one location and using public transportation for day trips.
My daughter and SIL loved Porta. Fatima?? When in doubt, purchase a RS guidebook.