When you download an APK file, you're essentially bypassing the protections offered by Google Play. This means that the application may have been modified to include malware before it was installed on your phone. To protect yourself, make sure you only download APKs from trusted sources like APKMirror. This site has practically all the APKs of the applications you're looking for, and it's constantly adding new versions. All APKs on the site are completely free and safe to download.
Adware can make the user interface annoying at best or unusable at worst, and malware that spreads through APK files can spy on you and steal sensitive information, such as passwords and personal files. To protect yourself from malicious applications, you should always scan an APK file for viruses before installing it. You can use HashDroid to verify the MD5, SHA-1, and SHA-256 of an APK file.
Google has made it even easier with the App Sharing feature, which allows you to send or receive APK files via Bluetooth directly from the Play Store. If you need to install an application from a third-party source, try running the file APK by scanning for viruses before installing it to make sure it's (hopefully) safe.
In conclusion, APK files are a great way to install applications on your Android device, but they can also pose a serious security risk. To ensure your safety, make sure you only download APKs from trusted sources like APKMirror and always scan an APK file for viruses before installing it.