You can use SendSimpleEmail OOTB activity to send emails to the user and while passing the parameters, pass SendAllAttachments parameter to include all the Work Object attachments to the email or you can pass AttachmentCategoriesToSend parameter to send the attachments by category.
1.) add "Send email" smart shape after flow-action that contains the section you are submitting a pdf.
2.) In the configuration of smart shape (double-click smart shape), fill out who you are sending to as a property reference that holds the email value or hard code the email address. (If desired add subject. You also have option to add your correspondence rule here if you have one.)
3.) After Subject input and Message input, there is checkbox for "Include Attachments", check that box.