It's not an option to run a Task from the Contains tab.
Are you saying you need to create PDF drawings on all the old slddrw files, even if they are not being changed? Obviously that is going to take a while even with a Task.
You can run tasks on Search Results. Hence if you can set up a search that will capture what you need then you can generate your PDF from there. In this case needing to do all the old drawings is actually easier in some ways.
If you only want to run it for drawings of certain assemblies (and all their components). Then I would like to know how to do that too. Sorry.
By the way, I agree it is extremely lame that we can't do anything from the Contains or Where Used tab. Someone started an Enhancement Request on this (please vote on SPR 529670). It is a cruel joke that they have the Right Click>> Select Files thing when you can't do anything after you select the files.
You could create a transition to a temporary state that would trigger the task. Then you could select entire folders or vaults. From the temporary state create a transition that will automatically return to the original state.