Plugin crashing when trying to update pdf

There is a critical error every time I update or save my pdf file using this plugin.

Error Details ============= An error of type E_ERROR was caused in line 301 of the file: /wp-content/plugins/tnc-flipbook-3d-lite/includes/class-tnc-flipbook-3d-text-extract.php Error message: Uncaught Error: Call to undefined function shell_exec() in /wp-content/plugins/tnc-flipbook-3d-lite/includes/class-tnc-flipbook-3d-text-extract.php:301 Stack trace: #0 /wp-content/plugins/tnc-flipbook-3d-lite/includes/class-tnc-flipbook-3d-text-extract.php(98): TNC_Flipbook_3D_Text_Extract::extract_with_pdftotext() #1 /wp-content/plugins/tnc-flipbook-3d-lite/includes/class-tnc-flipbook-3d-text-extract.php(55): TNC_Flipbook_3D_Text_Extract::extract() #2 /wp-includes/class-wp-hook.php(341): TNC_Flipbook_3D_Text_Extract::maybe_extract_on_save() #3 /wp-includes/class-wp-hook.php(365): WP_Hook->apply_filters() #4 /wp-includes/plugin.php(522): WP_Hook->do_action() #5 /wp-content/plugins/tnc-flipbook-3d-lite/includes/class-tnc-flipbook-3d-admin.php(261): do_action() #6 /wp-includes/class-wp-hook.php(343): TNC_Flipbook_3D_Admin::save_meta() #7 /wp-includes/class-wp-hook.php(365): WP_Hook->apply_filters() #8 /wp-includes/plugin.php(522): WP_Hook->do_action() #9 /wp-includes/post.php(5182): do_action() #10 /wp-includes/post.php(5295): wp_insert_post() #11 /wp-admin/includes/post.php(449): wp_update_post() #12 /wp-admin/post.php(227): edit_post() #13 {main} thrown