Is there any rewrite rules in place?Check network calls on click of link from external site.Share the error log if possible.

Use query builder Refer cheatsheet https://github.com/paulrohrbeck/aem-links/blob/master/querybuilder_cheatsheet.md  

You can use content as a service to get the data from AEM .

Restart your AEM instance and it should fix itRef : https://experienceleaguecommunities.adobe.com/t5/adobe-experience-manager/installed-6-5-4-sp-rrd-already-closed-exception/td-p/356891 

Ref: https://experienceleague.adobe.com/docs/experience-manager-65/developing/introduction/reference-materials.html?lang=en one-inside.com/aem-headless-cms/#Content_APIs 

https://github.com/adobe/aem-project-archetype/releases

Ref: https://sling.apache.org/documentation/bundles/models.html 

You can use as a viewer on a product list page where user can click on view details and pop-up will open with pdf viewer.

 Go to solution partner portal https://solutionpartners.adobe.com/solution-partners/home/applications/experience_cloud/experience_manager/training/dev.html Here you will get test questions after video tutorial.    

Refer : experienceleague.adobe.com/docs/experience-manager-65/authoring/siteandpage/page-analytics-using.html?lang=en