Do I need a .pdb file for mscorlib.dll?

I ran dottrace memory and I see that my app uses System.String more than anything else,

but at the bottom of the page it says 'cannot find PDB file for C:\Windows\Microsoft.NET\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscirlib.dll (browse)

Can I get the pdb file for this? I can see the path to the .dll when I browse but it only allows me to click on .pdb files, but I don't see a .pdb file for it.

This loading of the pdb file would allow me to see the source code I assume, do I need or want to see it with dottrace memory? Will it help me?

my application is a wpf app that opens dockable windows inside the application, it then does a bunch of other stuff.

The memory issue I'm trying to resolve is what methods and/or types are using up large amounts of memory.

I see large amounts of memory being consumed when a dockable window is opened but when it gets closed the memory doesn't get cleaned.

I don't think its a leak because I can empty the gc and the memory footprint of the application goes back down.

I'm just trying to optimize the application so it doesn't use so much memory.