I been taking a good look at what i mentioned in my previous post, and i changed my mind a bit... long story short, i dont think the .pdb files are stored in the XMB databases
The reason why im saying this is because there is not any
database codename related with them, neither the
xCB functions (used to acess the database)
So... doesnt seems posible to scan the database searching for pdb files... and the reason for that most probably is because the databases doesnt allows to store info about the .pdb files... as simple as that

But something worthy to be mentioned is the .pdb files itselfs are working a bit like an "extension" of the database... in some way it looks over 90% of the info stored in the .pdb files have a direct relationship with the database "fileds" (available to store that kind of info)
Actually, what happens is when the download is completed (and the pkg installed and indexed or whatever file you downloaded, could be just a video)... at that point most of the info from the .pdb is copypasted to the database
Something really interesting is the ID's used in the .pdb files should have a direct relationship with the "fields" of the database, i mean the ID's that appears in psdevwiki
https://www.psdevwiki.com/ps3/Project_Database_(PDB)
And the ones mentioned in this thread
https://www.psx-place.com/threads/pdb-research-on-background-downloads.16408/
That IDs have an official codename that can be seen in the database fields, as example, in the wiki page there are a few added by
@mysis
Video:Video.reserved30
Video:Common.reserved24
Video:Video.reserved29
This is exactly the kind of paths (and the accurate codenames) that needs to be used to access the database contents

You can see how this kind of paths are used in the:
https://www.psdevwiki.com/ps3/XMBML_Files
https://www.psdevwiki.com/ps3/Registory.xml
I wonder how he found them though, lol... im trying to imagine it because (incase they appears in some database file) is posible to identify all them
All that IDs are actually optional, as pink1 proved, and this opens a few posibilities

The "failproof" way (what most people that was doing) is to use the same IDs than the PSN store when you download that kind of content
There is an alternative... what pink1 suggested... trying to remove the uneeded IDs
But there is another alternative... the fact is the databases reserves data "slots" for a lot of fields so we could add other fields in the .pdb (that are going to be copypasted into the database when the download is completed), this opens a lot of posibilities, keep in mind the pdb files are giving us access to update the database with other contents different than PKG (videos, photos, music, playlists, vmc, game patches, dlc, etc...)