Hope you are doing well and thank you for reaching out. As described, you want to send 1 email via outlook to multiple clients for signature.
1. Are you trying to get the signatures from multiple clients on a single PDF document?
If yes, please use the 'Request Signatures' option present in the 'Fill and Sign' tool (Powered by Adobe Sign) to send the document for signature and add the clients email in the recipient email list, in the order you want their signatures, so that once the first person will sign the document it will go to the next person in the list for signature. For more information please check the help page: https://helpx.adobe.com/acrobat/using/send-for-signature.html
2. Are you trying to get the signatures from multiple clients on their individual copy of the PDF document, at once?