Stabilize sharepoint search

[MrMarshall]

I have a customer with a lot of files in different libraries but all on the same modern sharepoint online site. 

We are talking about 1000 - 10000 files in each of the 10 libraries. Files usually exists of 90% .pdf files, but a few of them are "stranger" types like .dxf and .nc1.

From the beginning the search function has gone from bad to worse. It's rare that it's working at all. Misses document and most of the time it give zeros results back.

Have tried to reindex the site and each library for it self. No luck and still very unstable.

Is there anything I can try to stabilize the search function?

Might it be because of large number of files on a single Site? Or unusual filetypes that's messing with the index?

[#2]

@MrMarshall 
The file formats need attention. The content of dxf files is not indexed (only the filename). Possibly same applies to the nc1 files. Happy to test the indexing of nc1 on my 2 tenants. If you can share a sample file without sensitive or privacy information that would be great.

The number of files should not matter. Search is not affected by list view threshold problems.

About the pdf files: are they PDF files with text content or are they PDF files containing only images (e.g. output format of a scanner)?

Paul

[#3]

@Paul de Jong Hi! Yes, .pdf should it be.

Sorry for being unclear, I am not searching for the file content. It's enough to try searching for the names..

[#4]

@MrMarshall 
PDF clear. Indexing filenames for exotic formats like .nc1 should work. 

I just tested this on my tenant with a modern UI. and it does not work. then tried a bit later and it worked fine.

To check if the file is actually properly indexed use the search api. e.g. enter the following in a browser

https://<yourcompany>.sharepoint.com/_api/search/query?querytext=%27(<filename>*)%27

and replace <yourcompany> and <filename> by values for your environment.
It looks like the indexing and search is working OK but the modern interface for search is intermittently broken.

Paul