It should be possible to override the border color by adding something like this to your Theme's styles:
div.pdfemb-viewer { border: 1px solid red !important; }
To hide the border, you would use:
div.pdfemb-viewer { border: none !important; }
Of course, you might need to ask your web designer to help if you're not sure where to add that for your particular site.
The best arrangement is to have a child theme and to add the above code to the style.css there.
But the easiest method is to paste the code into Appearance -> Customize -> Additional CSS.
You can also add other CSS styles to change other aspects of the PDF Embedder, such as the button color.
To hide the toolbar, you could try:
div.pdfemb-toolbar { display: none !important; }