Recently, google custom search has been shutting down and ending their services come April 2018 (don't quote me on that).
In light of this, I've been attempting to move our Drupal site's search to a new search engine, namely Apache Solr.
Our drupal site hosts tons of files from PDFs to Images to JSON and XML files. I haven't had any trouble indexing these files since they're stored locally on the same machine that hosts the Drupal site, but we have a bunch of external files that I used to have no problem searching with GCSE.
I want to be able to index external files and be able to search/query them with Solr just like I was able to search them with GCSE.
Is this possible? I'm sort of a noobie and have been following step-by-step guides up until now in order to get Solr search up and running on our site.
If anyone has any idea on how to search and query external files with Apache Solr, I'd be grateful.
Yes, it's possible to index different external files inside Apache Solr. There are plenty of tutorials, how to do that.
I will recommend you to look through this reference guide. Basically most of the stuff under Indexing and Basic Data Operations, with paying attention to the Uploading Data with Index Handlers, which will help you to index XML/XSLT, JSON and CSV data and also take a look at Uploading Data with Solr Cell using Apache Tika, which will explain how to index PPT, XLS, PDF and others more complex formats.
On the side of the querying it - follow some initial guidelines from Searching, when you will have troubles - feel free to ask additional questions here.