1. The idea of "portfolio" is a collection. An artist would have a portfolio of his works to show someone who was interested. This marketing spiel is trying to make the metaphor that these hotels are great works of art.
2. I believe they are saying that each hotel is a landmark and it is filled with art that is interesting and unique. "Boutique" is a small shop, so the implication is that something you get at a boutique is not some mass produced item that you can buy anywhere.
3. Yes, "autograph" is "signature." You collect them by staying at the properties. The point is that each is unique, like each person's signature is unique. "Great" modifies "signature." Each person's is unique, but if you have Thomas Jefferson's signature that is greater than having the signature of some random 18th century person.
4. The Strip in Las Vegas. Home to the most famous casinos.
5. No, this is more marketing speak. Each hotel is "dynamic" meaning full of energy and exciting.
6. "We also have some cheaper hotels in those countries under the name 'AC Hotels.'"
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