Convert Multi-Column List to Single Column

functions.php

<?php

add_filter( 'gfpdf_pdf_field_content', function( $value, $field, $entry, $form, $pdf_field ) {
	if ( $field->type === 'list' ) {

		/* get out field value */
		$value   = $pdf_field->value();
		$columns = is_array( $value[0] );

		/* exit early if list field is empty */
		if ( $pdf_field->is_empty() || ! $columns ) {
			return '';
		}

		$columns = array_keys( $value[0] );

		/* Start buffer and generate a list table */
		ob_start();
		?>

		<table autosize="1" class="gfield_list">
			<tbody class="contents">
			<?php foreach ( $value as $item ) : ?>
				<?php foreach ( $columns as $column ) : ?>
					<tr>
						<th>
							<?php echo esc_html( $column ); ?>
						</th>
					</tr>

					<tr>
						<td>
							<?php echo esc_html( rgar( $item, $column ) ); ?>
						</td>
					</tr>
				<?php endforeach; ?>
			<?php endforeach; ?>
			</tbody>
		</table>

		<?php
		return ob_get_clean();
	}

	return $value;
}, 10, 5 );