My portable A/C unit has an odor when operating in cooling mode, like a musty smell. I did some research and understand that these units require maintenance just like the bigger split units in which usually the coils are cleaned. I removed the cover and was able to clean some dust off of one of the coils but wasn't able to access both sides of it. The musty smell still remains. I've heard that a gas leak can also occur and is something to be concerned about so I wanted to make sure this isn't what I am smelling also.
I know there is a product you can use to remove mold if it's in the ductwork, I've used it in a car I used to have that would have a moldy smell from the A/C in the summer. I don't remember the name but I got it at an auto parts store like Autozone/Advance Auto Parts. Buy it, turn the A/C unit on max and spray it into the intake and let it work through the whole system
Before doing that, I would also check that the condensor is draining properly and that you don't have a leak that is causing condensation to build up and not drain properly.