You do not need an illegal license key to use PDF-XChange Editor. Tracker Software offers a highly capable free version directly to the public.
Use the unactivated PDF-XChange Editor for viewing, adding comments, highlighting, and filling out standard forms. It remains completely free for personal and commercial use as long as you avoid the premium features that trigger watermarks.
It remains one of the lightest and fastest PDF readers available.
These premium features are not included in the free version and cannot be unlocked by a simple "free license key" found online.