In your title you say a "custom message". In the body you say a "canned memo". Not sure exactly what your intent is.

If you want the e-mail that the PDF is attached to to have some message (that is more or less the same for each invoice) then go to Paperless Office | Setup | Electronic Delivery Message Maintenance.

My goal is to send a "Letter / memo / note" to the customer along with a copy of the Sales order. Something that is personal and informing the customer that we have received your order and it is being processed.

I can send a invoice / Sales order etc using Paperless office, but how would i attach a pre-formatted letter / memo / note along with the copy of the invoice / Sales order / Purchase order?
I got some ideas using Crystal reports, but was not sure if the office Merge has the function, and or paperless office?
I am exploring each of these right now, that is why i am asking for any type of suggestions.

Can you define "personal"?

If it is along the lines of "How's life treating you? Did you get that new car?" . You would probably have to put that directly in the data entry for the invoice which I suspect you really wouldn't want to do. Something like that would print via the crystal report design in the body of the invoice.

Alternatively, when you print the invoice. The printing window has some memo fields that would apply to every memo in that batch. Those are printed on the invoice but not saved in the actual accounting database. Usually they are used for something like "Have a happy holiday."

If you are looking for something like "Thanks for your order {customer name}. Invoice no {invoice no} should be shipping to you shortly. If you have any questions, please call ...." then my original suggestion applies.

Cross over on the posts.

If it is more or less boiler plate you can edit the crystal report and put all info in the invoice report header and then do a new page after so that the invoice comes out on a second separate page.

Had one client who on purchase orders put all their legalize in the report footer ( a full page of stuff) with a new page before.

Keep in mind that when you edit forms in CR you will want to do your extra page in a Group header / footer section instead of Page / Report sections, because forms are designed to work when printing batches of multiple transactions at once.