I think there are more than few archival strategies that can be considered depending upon how frequently the archived data will be used in future and what techniques are acceptable for data access.
Not going into much detail in absence of the landscape, few simple approaches that you can consider as below.
If archived data will not be accessed by a Pega application and is required for regulatory or audit purpose only, then the best approach would be to use Pega BIX to extract the data in flat/relational structure so that data can be queried easily when required.
If the data will be accessed by a Pega application then you may want to take a cold backup of the data and enable incremental backups to refresh the archive on a suitable frequency. Use Pega activities to purge inactive data from online application. In this approach, there will not be a clean separation in online and archived data, as archive copy will have additional data unless you purge data from archive as well.