How to find corresponding .pdb inside .NET assembly?

You can use the dumpbin tool from a Developer Command Prompt, e.g. a cmd line like this

dumpbin /HEADERS YourAssembly.exe   

would show the path to the PDB file in the Debug Directories section similar to this

Microsoft (R) COFF/PE Dumper Version 14.00.24213.1
Copyright (C) Microsoft Corporation.  All rights reserved.


Dump of file YourAssembly.exe

...


  Debug Directories

        Time Type        Size      RVA  Pointer
    -------- ------- -------- -------- --------
    570B267F cv           11C 0000264C      84C    Format: RSDS, {241A1713-D2EF-4838-8896-BC1C9D118E10}, 1,  
    C:\temp\VS\obj\Debug\YourAssembly.pdb

...