If you need to embed a PDF found on the library or elsewhere, and you want just a simple snippet of HTML code, add to the HTML code of the page:

<object width="400" height="500" type="application/pdf" data="https://theanarchistlibrary.org/library/example.pdf">
    <p>Insert your error message here, if the PDF cannot be displayed.
    I suggest you to insert the link to the pdf</p>

See the MDN docs for the HTML <object> element for more details and supported browsers.