Most save files are stored as plain text, JSON, or XML. Once you decompress the .zip , you can use any basic text editor to view them.
Right-click the .zip file and select Extract All... . Once extracted, right-click the file you want to see (like a .sav , .json , or .txt ) and select Open with > Notepad . Saves(1.0.4).zip
This mod for SCP: Containment Breach uses a .zip with that exact name. If this is the one, you can edit unlockables by finding the save files in your game directory. Most save files are stored as plain text, JSON, or XML