Hi Ishitaa,
To see your already created portfolio's performance reports either a) you can click on the 'Search/Display/Social section of UI on the top - then under Optimization - select Portfolio's link' and see the data on UI itself or download it as a report by clicking the 'download' link on right side under the date range. or b) you can visit the 'Insights and Reports' section of Search/Display/Social depending upon your campaigns and run a 'Portfolio report' under 'Basic reports' link from the 'Create report' part option. There are other performance related reports available too for your reference under the same 'Create report' link.
Hope this helps.
Regards
Nidhi
