The reasons for such problems vary and if you search the web for 'corrupted downloads' you will find an extraordinary number of related pages. This is definitely a global problem which usually includes the following:
- Network and connectivity issues and / or interruptions.
- Apache / Web scripts settings - by default the web protocol (HTTP) is NOT designed for handling large data transfers. This means that every webserver is optimized for faster page rendering instead of reliable file transfers. Furthermore there are many configuration timeouts and limits to prevent heavy and long web operations which might overload the webserver and slow it down.
- Browser problems - some web browsers, especially Microsoft's Internet Explorer, have problems with handling large file downloads.
The only way to have a reliable web download is by using a download manager. SiteGround recommends TrueDownloader - it is free and open source.