Best way of doing it is create a new folder in inbox and move all mails you want to archive from all other folders in this folder (say the old chunk of 8 GB out of total 10 GB). Now go to File import and export export to a file click on ‘next’ select ‘Personal Folder File (.pst) click on ‘next’ select this folder click on ‘next’ give appropriate path and name of the .pst file by clicking on ‘browse’ under ‘save exported file as’ select ‘replace duplicates with items exported’ click on ‘finish’ choose appropriate encryption settings and give password that you want click on ‘ok’ you get a .pst file.
Now you can delete this folder from your outlook inbox. And automatically your original usable .pst file reduces to say 2 GB. Later as and when if required you can restore back the separately saved .pst file.
While exporting if you don’t want to save it as .pst, you have many options like export it to excel, access, .csv etc.