An example of how to include a custom link to the 'actions' column of the My Events section based on the check in status.

example.php

<?php //Please do not include the opening PHP tag if you already have one 

//Add a link for only checked in registrations.
function tw_add_checked_in_file_to_my_events( $actions, $registration) {
	//Check we have an EE_Registration object.
	if ( $registration instanceof EE_Registration ) {

		//Pull the 'latest' datetime (sorted by date) from the registration.
		$latest_related_datetime = $registration->get_latest_related_datetime();

		//Check we have an EE_Datetime object.
		if( $latest_related_datetime instanceof EE_Datetime) {
			//Pull the checked in status for the current registration using the datetime ID we just pulled in.
			$checked_in_status = $registration->check_in_status_for_datetime( $latest_related_datetime->ID() );

			//Check the checked in status.
			if( $checked_in_status == EE_Checkin::status_checked_in ) {

				//Set the file URL to use when the user is logged in.
				//This example links to Google, you could use an event meta value.
				$file_url = "https://google.com";

				//Add the checked in file link to the $actions array.
				$actions['checked_in'] = '<a aria-label="' . __( 'Link to checked in file', 'event_espresso' ) . '" href="' . $file_url. '">'
							. '<span class="dashicons dashicons-media-archive ee-icon-size-18"></span></a>';
			}
		}
		
	}
	return $actions;
}
add_filter( 'FHEE__EES_Espresso_My_Events__actions', 'tw_add_checked_in_file_to_my_events', 10, 2);

//Add a checked in icon to the my events legend.
function tw_add_checked_in_file_to_my_events_legend( $items ) {
	$items['ticket'] = array(
		'class' => 'dashicons dashicons-media-archive',
		'desc' => esc_html__( 'Checked in file', 'event_espresso' )
	);
	
	return $items;
}
add_filter( 'FHEE__status-legend-espresso_my_events__legend_items', 'tw_add_checked_in_file_to_my_events_legend');