Invalid file type when using wp_upload_bits to upload PDF to a custom post type

I'm trying to upload a PDF file for a custom post type called Products using wp_upload_bits. I'm only doing this in my wp-admin. I don't want these files added to the media library, I just want to upload them and return the URL for when I display the Product.

However when I do I get the error that it is an invalid file type returned from the function. I've set the enctype using this in my functions.php

//Allow file uploads
function update_edit_form() {
    echo ' enctype="multipart/form-data"';
} // end update_edit_form
add_action('post_edit_form_tag', 'update_edit_form');

Here is my code for generating the metaboxes on the admin menu

function products_pdf_uploads_show_meta() {
    global $meta_box_pdf_uploads, $post, $prefix;

    echo '<table class="form-table">';
    echo '<p class="description">Upload your PDFs here</p>';
    foreach ($meta_box_pdf_uploads['fields'] as $field) {
        // get current post meta data
        $meta = get_post_meta($post->ID, $field['id'], true);

        echo '<tr>',
                '<td>',
                    //'<input type="file" style="width: 700px;" name="', $field['id'], '" id="', $field['id'], '" value="', $meta ? $meta : null, '" />'
                    '<input type="file" style="width: 700px;" name="', $field['id'], '" id="', $field['id'], '" />',
                '</td>',
             '</tr>';
    }

    echo '</table>';

}
add_meta_box($meta_box_pdf_uploads['id'], $meta_box_pdf_uploads['title'], 'products_pdf_uploads_show_meta', $meta_box_pdf_uploads['page'], $meta_box_pdf_uploads['context'], $meta_box_pdf_uploads['priority']);

It doesn't have anything to do with the last line does it? Where I'm calling add_meta_box and passing the custom post type of PRODUCT and not specifiying PAGE or POST? Just a though.

Here's my code for uploading the metabox PDF file:

   //Upload PDF files
    foreach ($meta_box_pdf_uploads['fields'] as $pdf_field) {

        //put file array into a variable
        $pdf = $_FILES[$pdf_field['name']];

        //if array is set and there is no error 
        if(isset($pdf['error']) && $pdf['error'] > 0) {

            //setup error handling based on error code
            wp_die('Error uploading file: Error Number is ' . $pdf['error']);

        } else {  //Passed              

            //setup file type allowed
            $supported_file_type = array('application/pdf');
            //Get the file type
            $uploaded_file_type = $pdf['type'];

            //check if file type is allowed
            if(in_array($uploaded_file_type, $supported_file_type)) {

                //upload file
                $uploaded_file = wp_upload_bits($_FILES[$pdf_field['name']], null, file_get_contents($_FILES[$pdf_field['tmp_name']]));

                print_r($uploaded_file);
                wp_die('did it upload?');

                add_post_meta($post_id, $pdf_field['id'], $uploaded_file);
                update_post_meta($post_id, $pdf_field['id'], $uploaded_file);

            } else {  // NOT A PDF

                wp_die('The file you tried to upload was not a PDF');

            }


        }

When I do a print_r($uploaded_file) to see what the function returns I get Array ( [error] => Invalid file type )

I've checked my wp-includes/functions.php and it apparently allows the file types under the get_allowed_mime_types() function.

'pdf' => 'application/pdf',

Am I missing something?

Here is the output from var_dump($_FILES). I currently have two fields, when I'm testing I'm only selecting a file to upload for the first one.

array(2) { ["aps_pdf1"]=> array(5) { ["name"]=> string(12) "1testpdf.pdf" ["type"]=> string(15) "application/pdf" ["tmp_name"]=> string(14) "/tmp/phpwdXK0T" ["error"]=> int(0) ["size"]=> int(8278) } 
["aps_pdf2"]=> array(5) { ["name"]=> string(0) "" ["type"]=> string(0) "" ["tmp_name"]=> string(0) "" ["error"]=> int(4) ["size"]=> int(0) } }