Error when activating

Since version (2.8.4) Download Manager requires php v5.3.0+ ( https://php.net/manual/en/language.namespaces.rationale.php ), may you please update your server php version, however if that is not possible, you can get back the previous version from here https://downloads.www.ads-software.com/plugin/download-manager.2.8.3.zip

I have version 5.4.2 installed. Any other ideas?

You only will show the error, when installed php version doesn’t support “namespace” ( https://php.net/manual/en/language.namespaces.rationale.php ), so, may you please contact with your hosting company, why installed php version is not supporting namesapce when it 5.4+

if i downgrade how can i save the files? do i need to back up and specific file? cause it is not activating at all and i dont want to loose my files

You can delete plugin files from ftp, it will not harm your old data.