You should choose your e-invoicing mode based on your internal organisational structure, your information system and your client's constraints. 

Choosing PDF without signing them implies that you are able to establish a reliable audit trail between the invoice and the delivery of goods or the provision of services, and to put in place controls documented within your internal organization for this purpose.

Choosing signed PDFs exempt you from implementing an audit trail since CEGEDIM signs the PDF and files the electronic invoice.

The e-invoicing mode should be chosen before switching your invoicing flow to electronic mode.