A popular app that lets you sign APK files directly on your phone or tablet. It supports modern V2 signing schemes and allows you to manage keystores.
Whenever possible, download from the Google Play Store to minimize the risk of malware. Download Yaps209 sign apk
If you are looking for authentic, untouched apps, APK Mirror is a highly reputable source that verifies all signatures before publishing. Safety Checklist for Unknown APKs A popular app that lets you sign APK
If you are looking for a reliable way to sign APK files on Android, What is "APK Signing"? What is "APK Signing"?