It depends on the underlying language that the developers used.
For Java and .NET languages it is possible most of the time to "de-compile" the source code. it's not going to be the true original code (variable names will be different, optimizations by the compiler will have rearranged code, etc) but it will be close.
Other languages such as C/C++ are not so easy.
Please refer to this detailed discussion here:
https://stackoverflow.com/questions/2659513/how-to-get-source-code-of-a-windows-executable
Best regards.
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