I have done sufficient testing with PDF files and .txt files to be certain that text indexing that happened to old files is no longer happening with new files. 

Discussed in GMF at https://groups.io/g/GroupManagersForum/topic/76990562

I am the owner of a group used for a condo association. The group was started on 03/19/19 so is grandfathered in to have more features than newly created groups.  We have stored files in folders in the file section, including folder for "Board Minutes" with sub folders for each year.  Board minutes are stored as "Print to PDF" files and are searchable.
 
I open a PDF file that was uploaded last year and find an uncommon word in it.
I search for the word using the PDF reader and it is found.
I search for the word using the groups.io Files/Search and the file shows up in the results

I upload a new file (created from Print to PDF from MS Word) with the word "abracadabra" in it.
I search for the word abracadabra using the groups.io Files/Search and the file does not show up in the results.

I can confirm the same issue, regardless of extension, albeit in my case the old vs new distinction is not as clear, i.e. I can search for the keyword that exists in two TXT files but only one is returned and in a particular case it finds a newer one, but (and it may be a clue), the newer one that is found was updated at some point in time.

It almost sounds to me like a search index needs rebuilding or something.

Cheers,
Christos