Always you should setup credentials stored securely in the report server for the report before you try to setup subscriptions to send report email with pdf format.
Report Manager, select your report. Click the Properties tab, then select 'Data Sources' on the left side of the page. Make sure the 'A custom data source' is the radio button selected, then click the radio button for 'Credentials stored securely in the report server'. Type in your credentials to run the report and if it's a domain account, then check the box for 'Use as Windows credentials when connecting to the data source'. Click Apply at the bottom and you should be able to create a subscription.
Report Manager, select your report. Click the Properties tab, then select 'Data Sources' on the left side of the page. Make sure the 'A custom data source' is the radio button selected, then click the radio button for 'Credentials stored securely in the report server'. Type in your credentials to run the report and if it's a domain account, then check the box for 'Use as Windows credentials when connecting to the data source'. Click Apply at the bottom and you should be able to create a subscription.