SUMMARY
Baloo ranks the relevance of results undesirably. For example, including the entire exact title of a pdf book can produce many seemingly irrelevant results but not the intended result (at least not visibly in the selection of only the most relevant results shown in the start menu).

STEPS TO REPRODUCE & OBSERVED RESULTS
I have pdf's of many textbooks on my filesystem. I can search contents of the title, e.g. "Advanced Engineering Electromagnetics" and about 10 results appear--none of which are the intended textbook with the file name "C A Balanis - Advanced Engineering Electromagnetics (Second Edition)John Wiley and Sons (2012).pdf". The first result is actually "Advanced Window Behavior" which should not be even listed in the results given that the words "Engineering" and "Electromagnetics" are also included in the search. 

If I can get a textbook I search for to appear, it is often the lowermost result which would indicate that Baloo believes it is the least relevant. Indeed, the textbooks that Baloo believes more relevant potentially have the words "engineering" or "electromagnetics" or "advanced" within them. Nevertheless, in my opinion most of these results should instead be calculated to be irrelevant to the search. But in any case the file with the all of the searched words in the filename (not to mention in the same order with the same casing) should appear as the first result.

This is just an example; many file searches are like this. Including almost every word in the textbook title and the authors' names does not necessarily result in the desired result appearing at all (let alone at the top), say if these names are mentioned in other files even in places different from the other query words even referring to other people with the same name.

Gnome Tracker demonstrates the desired behavior for these kinds of searches. But I must say that the intra-file searching capabilities of Baloo are impressive. Also, Baloo appears to search the pdf metadata whereas Gnome Tracker does not.

EXPECTED RESULT
I propose that you make the fact that several query words match some files' names or pdf metadata grant those files higher relevance than files that merely contain all of the query words at some place within them.

Secondarily, I propose you also ensure that successive word matches are given priority over dispersed word matches within a file (I do not know if this is the case or how it would be implemented). Successive matches within a file may approach the relevance level of dispersed matches within the file name or pdf metadata. The distinction between exact matches and dispersed matches may not matter as much when both are within the file name or pdf metadata. 

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux
(available in About System)
KDE Plasma Version: 5.18.4
KDE Frameworks Version: 5.68.0
Qt Version: 5.14.2