To use mail merge you would create an Excel worksheet that lists the recipient details, the Invoice Numbers, the PO Numbers, Company Names and Full Paths of the PDF files, all on one row for each invoice number.



The chances of extracting the data from the PDF files to the worksheet (or to the email as your original suggestion) are slim, and impossible without access to a sample.

Configure your email body as a Word document and insert the fields as required. Then, using the add-in, merge the data with the document to the body of an email and select the fields for the attachment.
The use of the process is documented on my web site and there is plenty of context sensitive help.

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Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com