How to create a shortcode for embedding pdf in iframe?

It depends on how you want the pdfs added but this should probably be a cover all:

function embed_pdf_files( $atts, $content = null ){

    //add default attributes here.
    $defaults = array(
        //'width' => '100%',
        //'height' => '1000'
    );

    //This overwrites defaults with the attributes in shortcode
    $a = shortcode_atts( $defaults, $atts );

    $src = $a['src'];

    //If no src present, get src by attachment id
    if( ! $src && $a['attachment_id'] ){
        $src = get_attachment_link( $a['attachment_id'] );
    }

    //If no src or content (srcdoc), return nothing.
    if( ! $src && ! $content ){
        return '';
    }

    //Comment out/in the attributes you want to allow the editor control over
    $html_attrs="";
    //src
    $html_attrs .=  $src ? ' src="' . esc_attr( $a['src'] ) . '"' : '';
    //srcdoc
    $html_attrs .= ( ! $src && $content ) ? ' srcdoc="' . esc_attr( $content ) . '"' : '';
    //width
    $html_attrs .=  $a['width'] ? ' width="' . esc_attr( $a['width'] ) . '"' : '';
    //height
    $html_attrs .=  $a['height'] ? ' height="' . esc_attr( $a['height'] ) . '"' : '';
    //seamless
    $html_attrs .=  array_key_exists( 'seamless', $a ) ? ' seamless' : '';
    //name
    //$html_attrs .=  $a['name'] ? ' name="' . esc_attr( $a['name'] ) . '"' : '';
    //sandbox
    //$html_attrs .=  $a['sandbox'] ? ' sandbox="' . esc_attr( $a['sandbox'] ) . '"' : '';

    //These are the global html tag attributes - remove any you don't want editable
    $global_attrs = array( 'accesskey', 'class', 'contenteditable', 'contextmenu', 'dir', 'draggable', 'hidden', 'id', 'lang', 'spellcheck', 'style', 'tabindex', 'title' );

    for( $i = 0; $i < count( $global_attrs ); $i++ ){
        $html_attrs .=  array_key_exists( $global_attrs[$i], $a ) ? ' ' . $global_attrs[$i] . '="' . esc_attr( $a[ $global_attrs[$i] ] ) . '"' : '';
    }

    return '<iframe' . $html_attrs . '></iframe>';

}
add_shortcode( 'embed_pdf', 'embed_pdf_files' );

This should be in your functions.php or in your own plugin or somewhere similar.

I commented out the attributes that can be potentially hazardous, but if you need them, they are there.

The editor can then use something like this:

…

[iframe class=”my-iframe” src=”http://example.com/mypdf.pdf”][/iframe]

…

or

…

[iframe id=”my-iframe” attachment_id=”193″][/iframe]

…

or

…

[iframe class=”my-iframe”]<p>My paragraph in an iframe</p>[/iframe]

…

References: W3C Wiki on iframe element, WP codex on shortcode api.