That method of embedding PDFs is deprecated -- here's some recommendations:
And here's more info on embedding:
To use the example above, you would use this code within an HTML widget, which appears to to display in Chrome and Firefox:
<!DOCTYPE html>
<html>
<head>
<title>PDF Example by Object Tag</title>
</head>
<body>
<h1>PDF Example by Object Tag</h1>
<object data="${resourcesFolderName}/filename.pdf" type="application/pdf" width="100%" height="500px">
<p>Unable to display PDF file. <a href="${resourcesFolderName}/filename.pdf">Download</a> instead.</p>
</object>
</body>
</html>
If the PDF is a single page PDF, you may need to uncheck 'Automatically optimize when exporting' when the PDF is selected in your resource library.