Are you trying to that you developed, or are you trying to re-sign a modified app for personal use? apk-signer - Apps on Google Play
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
If you are looking for a reliable way to sign APK files on Android, What is "APK Signing"? Download Yaps209 sign apk
Whenever possible, download from the Google Play Store to minimize the risk of malware.
Use VirusTotal to upload the file and scan it against dozens of antivirus engines. Are you trying to that you developed, or
Downloading APKs from unverified third-party sources can lead to malware installation , data theft, or device damage. Always be cautious with files that have obscure names.
Be wary if the app asks for excessive permissions, like access to your contacts or SMS, that it doesn't need for signing files. Whenever possible, download from the Google Play Store
The official command-line tool provided by Google for developers to sign and verify APKs.