For the average user, the best advice is: Do not attempt firmware updates from unverified sources. Do not enter random lock codes. And always keep a backup of your contacts via the "Copy to SIM" feature.
Here is a breakdown of the common variants: Nokia 105 Pac File
If you or a previous user has set a security code (a common practice with feature phones) and forgotten it, a hard reset isn't always enough. Flashing the full firmware via a Pac file will wipe the phone completely, removing the security lock in the process. For the average user, the best advice is:
Before you download anything, it is crucial to understand the anatomy of a PAC file. Here is a breakdown of the common variants:
There is no such thing as a "Nokia 105 PAC file" that you can install or configure. If someone is offering one, it is either a misunderstanding or misleading.
Resets forgotten security codes or pin locks.
PAC files are copyrighted by HMD Global / Nokia. Distributing them for commercial profit (selling on eBay) is piracy. However, downloading for personal repair is generally tolerated.