Basically what I want to do it this: a pdb file contains a location of source files (e.g. C:\\dev\\proj1\\helloworld.cs
). Is it possible to modify that pdb file
It is certainly possible, as On Freund has already pointed out. But if it is only so that the sources can be located and loaded during debugging, then a better way would be to set the source path correspondingly. Once set in a debugger, it will preemt all hard coded paths inside PDBs.
In windbg (for instance):
.srcpath+ path_to_source_rootor this (in case you're debugging remotely):
.lsrcpath+ path_to_source_root