PPF has a lifespan and will yellow/thin/dull. It also has a period in life where it's easy to remove, and then as it ages after it becomes more difficult. Depending on the quality of the film this can be from 3 to 5 years.
That was a great question actually and one that I've struggled with in the past. When using the wrong product you can make things very difficult for yourself. I get stuff like compounds, polish, and wax stuck there all the time, but a good microfiber and using a toothpick or your nail to press with can clean it out well. Just don't use any liquids or solids that dry quickly and if wiping after applying a synthetic wax, get one that has easy residue to remove. The Adam's butter wax, nufinish, opticoat, and the older chemical guys m sealer are all great products that I find super easy to clean up... or in the case of opticoat there is no clean up. If you ceramic coat it, you can go right over the ppf if it already exists, but it's preferable to remove first for paint correction and protecting the paint from the ppf layer (some installers claim this can extend your ppf working life, but I have no idea if that's true).