The PDB file needs to be in the GAC for debugging in Visual Studio. You can add this file through a feature manifest file, but you are unlikely to want to deploy the PDB to the GAC on your production server, so a better way on the dev server is to directly place files in the GAC. To do this, map a drive in windows explorer to

\\[your server name]\c$\windows\assembly

This bypasses the windows explorer shell special handling for this folder, and allows you to copy and paste files as normal without using GACUTIL.

In the GAC_MSIL subfolder, find the subfolder corresponding to the name of your assembly. Below there will be one or more version subfolders. Find the latests version subfolder., and copy your timer job dll and pdb file (built in debug mode!) to this folder.