Foul smell from the A/C means dead insects, bacteria, or most commonly the growth of fungus. To combat it, abroad you can buy charcoal filter but AFAIK it is not available in Pakistan. You can wrap a piece of coal in tissue paper and put it in the duct somwhere, a common ‘ghareloo totka’ usually done with fridge & depp freezer.

Occasional replacement of cabin filter is also important.