If you are using the elementor products widget, then for that you have to select the products to search and not the PDFs.
If you use the plugins onwn results container (similarly as it is in the tutorial and demo), then there is a chance. If the PDFs were uploaded to the products as attachments, then by changing this option WordPress will return the parent URL, which should be the product. Therefore the plugin will display the PDFs as results, but link to the product page itself.