A family of Microsoft word processing software products for creating web, email, and print documents.
After you save the certificate as a Word document (*.docx), use either the File > Print function (select a PDF "printer" such as "Microsoft Print to PDF") or the File > Save As function (set the document type as PDF before clicking the Save button). Send the PDF version to the customer. Keep the Word document version so you can make adjustments, and also save them as PDF files to send out.
Be aware, though, that PDF files created from Word documents can also be opened in Word or other software and modified. In fact, there is no way to prevent changes to anything you send to another computer, even if that requires retyping the document from scratch.