If you're doing business with them I wouldn't think you would be unknown. I've never had a problem with this unless you're trying to blanket "cold call" via email, I would think they would know they do business with you and click it. Regardless though, there's not really going to be a pretty way to put the invoice inside the body of the email (formatting at least to look like an invoice with different email providers and resolutions.). You can call the values though and say something like

Hello (Customer Name), your total due for (Invoice Number) is (Invoice Amount).

More complicated but you could also list the items included in the invoice with a recordset, but I don't think you're going to get it to "look" like an invoice nearly as good as a PDF.

How about an email template? I've seen somewhere that the Email function can use a template?

PDFs are generally considered safe and most companies will allow them. Your only alternative would be to write out the invoice using HTML, plain, or rich text. I cover that in my Email Seminar.

Ever seen the hacking demo at my old company, got in with a pdf?

Seriously? Wow.

All companies big and small that I've worked with in my 20+ years in wholesale & retail accepted (1) paper invoices by mail, (2) invoices as email attachments, and/or (3) electronic invoices sent straight to their systems.  In the 1990s I started seeing more and more companies choosing (3) to speed things up.  Some even required it, and accepted paper or emailed invoices with a penalty to the sender.  So I'm pretty sure email attachments are okay with most customers, but you just have to ask them what they require.  Different people have different requirements, and some customers are pretty fussy about how to do things.

Regarding sending an embedded image of an invoice, sometimes those images are blocked by the email app, so it's not totally reliable.  You just have to ask the recipient in advance to put you in the trusted senders list.