how do i embed the pdf gallery in wordpress post

You can use the Custom URL field, to modify the attachment link for each one of your gallery images:

where this is supported by the following plugin:

<?php
/**
 * Plugin Name:   Custom Attachments Links
 * Description:   Adds a new media field, to override the default attachment links.
 * Plugin URI:    http://wordpress.stackexchange.com/a/176668/26350
 * Plugin Author: Birgir Erlendsson (birgire)
 * Version:       0.0.1
 */

add_filter( 'attachment_fields_to_edit', function( $fields, $post )
{
    $fields['wpse_custom_attachment_url'] = array(
        'label' => 'Custom URL',
        'input' => 'text',
        'value' => get_post_meta( $post->ID, 'wpse_custom_attachment_url', true ),
        'helps' => "Override the attachment's default",
    );
    return $fields;
}, 10, 2 );

add_filter( 'attachment_fields_to_save', function( $post, $attachment )
{
    if( isset( $attachment['wpse_custom_attachment_url'] ) )
        update_post_meta( 
            $post['ID'], 
            'wpse_custom_attachment_url', 
            $attachment['wpse_custom_attachment_url'] 
        ); 
    return $post;
}, 10, 2 );

! is_admin() && add_filter( 'attachment_link', function( $link, $pid )
{
    if( $url = get_post_meta( $pid, 'wpse_custom_attachment_url', true ) )
        $link = esc_url( $url );   
    return $link;
}, 10, 2 );