Hiding the embedder's border or changing color/styles

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;
}