It depends. There’s two types of shadowless pack - the long seal and the short seal. The short seal has a chance of containing shadowless packs, but not a guarantee. Every documented long seal pack, on the other hand, has been found to contain shadowless cards. Most PSA graded packs have been weighed so it’s not advisable to open them if you have hopes of finding a holo card.
www.elitefourum.com/t/identifying-shadowless-unlimited-base-set-boxes-packs/16993/1