If you no longer have the installer files, Serif provides a dedicated portal for older versions. : You can redownload many installers from the Serif Legacy Downloads
Serif built Affinity from the ground up to be faster and more stable than the legacy "Plus" range. While legacy files (like .ppp for PagePlus) aren't always directly editable in Affinity without some conversion, the modern tools offer a significant leap in professional capability. 4. A Warning on "Free Key" Sites serif legacy product keys