Even though it is possible to download APK files for paid apps for free, it is generally not legal to do so and can be considered a violation of the DMCA (Digital Millennium Copyright Act) to use these apps without paying for them. Many developers rely on app sales as a source of revenue, and pirating apps by downloading them from unauthorized sources deprives them of this income.
There are some legitimate ways to obtain paid apps for free, such as through app giveaways or promotions, but these are typically offered by the developers themselves or through official channels, such as the Google Play Store.