We have a library with PDF files that generally have a file name structure like "PN YYYYYYY LN XXXX.pdf" where the Xs and Ys are numerical. There is no other relevant metadata. Searching for a PN (the Y numbers) will not always show all of the results. I can do a search for the LN (the X numbers) and it shows files that have the Y numbers previously searched for that didn't show in the initial search.
An example is this file: PN 1234567 LN TU1234 REC 5000075204.pdf
Searching for "1234567" is showing SOME results with the string IN the filename and also in the document. But not showing the file above. Searching for "TU1234" does show the file. Here are a few other searches I tried and their results:
"PN 1234567" - No
"PN 123*" - No
"PN 1234567 LN TU12*" - No (this one was really surprising)
"PN 1234567 LN TU1234" - Yes
"TU1234" - Yes