I don't think you can do this using the .NET Framework builtin functions, as they rely on physical files. However, there is a solution using the Mono Cecil library, as it has an overloads that takes a Stream as input instead of a file path for its symbols reader.

Here is an example of a Console app named "TestPdb" which dumps its IL code to the console, including PDB information:

using System;
using System.IO;
using Mono.Cecil;
using Mono.Cecil.Cil;
using Mono.Cecil.Pdb;

namespace TestPdb
{
    class Program
    {
        static void Main(string[] args)
        {
            // we use a Stream for the assembly
            AssemblyDefinition asm;
            using (FileStream asmStream = new FileStream("testpdb.exe", FileMode.Open, FileAccess.Read, FileShare.Read))
            {
                asm = AssemblyDefinition.ReadAssembly(asmStream);
            }

            // we use a Stream for the PDB
            using (FileStream symbolStream = new FileStream("testpdb.pdb", FileMode.Open, FileAccess.Read, FileShare.Read))
            {
                asm.MainModule.ReadSymbols(new PdbReaderProvider().GetSymbolReader(asm.MainModule, symbolStream));
            }

            TypeDefinition type = asm.MainModule.GetType("TestPdb.Program");

            foreach (MethodDefinition method in type.Methods)
            {
                Console.WriteLine("Method:" + method.Name);
                foreach (Instruction ins in method.Body.Instructions)
                {
                    Console.WriteLine(" " + ins);
                    if (ins.SequencePoint != null)
                    {
                        Console.WriteLine("  Url:" + ins.SequencePoint.Document.Url);
                        // see http://blogs.msdn.com/b/jmstall/archive/2005/06/19/feefee-sequencepoints.aspx
                        if (ins.SequencePoint.StartLine != 0xFEEFEE)
                        {
                            Console.WriteLine("  StartLine:" + ins.SequencePoint.StartLine + " StartColumn:" + ins.SequencePoint.StartColumn);
                            Console.WriteLine("  EndLine:" + ins.SequencePoint.EndLine + " EndColumn:" + ins.SequencePoint.EndColumn);
                        }
                        // etc...
                    }
                }
            }   
        }
    }
}

NOTE: since you only need to read from PDBs, you can recompile the Cecil library defining the READ_ONLY conditional symbol to save some bytes. You can also embed Cecil source code directly in your assemblies, no need to use the .DLL versions.