After completing this how-to you will have downloaded the certificate for the domain
Begin by browsing to the URL from where you want to download the certificate. Fort example readsoftonline.com
Begin with pressing the padlock. Then select the first option.
Select details from the certificate
Select copy to file
Go through the wizard and select DER choice. That way the certificate will still be encrypted
When you have gone through the wizard you will have saved the certificate on file, for other use.