Probably the most direct way would be to embed an iframe on the page and put the url of the pdf as the iframe source.
This might be a bit tricky to make work nicely (sizesetc) but for a simple pdf it might be ok.)
<iframe src="@@PDF_URL@@" width="600" height="800"></iframe>
You need to go to the source view of the html editor to enter this. And you would replace the @@PDF_URL@@ with the actual url of the PDF.
You might also see what Frankie Kam has done with PDFs and lightboxes. https://moodle.org/mod/forum/discuss.php?d=322120
(And as mentioned you can simplify and centralize the embedding of PDFs with either the Generico or VideoEasy filters.)