[a.mik]

Regarding pdf tools: 
I'm using pdf24 since quite some years (absolutely free of charge), and I'm very happy with it.
Yes, it lacks some in-page editing features.
But for all the page oriented tasks, like splitting into separate files or combining mutliple single files to one big it works perfect.

Regarding multiple instances of the same song: 
I would also propose the csv method, so you will have the file stored only once, but multiple independent songs in the database, one for each of the parts.
However, this would mean that you have to create this splitting on beforehand.
If your usecase is more like: at the final rehearsal it is decided which part you will need, then the way of copying and changing the used pages will be more useful.