Sage 100 does not require an email client like Outlook on the workstation.  The system handles the email directly to the email server, potentially in batches, and with that enhanced ability comes less control over customizing each individual message that goes out.

If you generate the Paperless PDF then find it in the Customer Viewer, there is an email function from that screen which might allow more runtime options.

What I do when I am in an invoice is to click the Quick Print and then Preview and then go to Adobe Application, click the email icon, enter in the email address and then enter in any message and send.

If you have more of a generic type of comment (and not something manually created for that 1 customer/vendor and that specific document) you may be able to use the Electronic Delivery E-Mail Message editor to Add standard text to body of email when emailing invoices, purchase orders, payroll direct deposits and vouchers. Electronic Delivery Message Maintenance (Paperless Office, Setup), 

Adding things like Dear Customer Name, Invoice #, and more can be added.

It will depend on what version of Sage 100 you are using as to how evolved the editor is, significant changes implemented in Sage 100 2019 (text formatting added, Size of fields, merge fields added, etc.).

Another option for personalized one-off emails, is to set your email address as the client default email, and send the PDF to yourself.  Then from your inbox you can send to your customer adding any specific comments at that time as well as any attachments, necessary.