Best way to search content from millions of excel files?

This is not an easy problem to solve. You mention that you don't want to use SQL, so we can assume you want to look at the actual contents of the Excel file. If you are talking about physically searching the contents of millions of Excel files then you are going to to have to wait a long time. There is no rapid way to do this.

There are document management systems that allow this to be done, but most are complex, expensive, and require a lot of hardware to run on.

You mention Solr which is a great tool for indexing and searching information. And using Solr Cell you are able to index the contents of file like Word, Excel, and PDf.

If you are confused the best advice to give is pick a tool like Solr and start learning. There are several great resources for this tool in particular. It took me a couple of days to first learn the basics and get a server setup, but once that was done I was able to move through the topics pretty quickly. We now use the tool to handle indexing of data for about 95% of our applications and in turn use that for searching.