Pak Loader is a plugin that can be used in Blueprints and C++. Due to how Unreal Engine works, .pak files are only intended to be loaded in packaged (shipping) builds.
Accessing the assets within the Pak file depends on how the .pak was created (paths, mount points, etc.) and what you are intending to use it for.
You can find an example project and an example .pak file in the link below.
Read the documentation to learn about mounting pak files. If you are a new to Unreal this plugin might not be suitable for you, as mounting pak files is a non trivial thing.

Features:

• Mount/Unmount .pak files.
• Register mount points to support references between pak assets.
• Download (.pak) files via HTTP.
• Load objects (all kind of assets).
• Load classes (Blueprints).
• Utility functions like GetMountedPakFilenames, GetFilesInPakDirectory, SHA1SUM, IsValidPakFile, etc. (see documentation)

Limitations:

• Only tested on Windows x64.
• Loading assets from mounted Pak files only works in packaged/shipping builds of your project. (Not in Editor)
• You can't load unsigned Pak files if your game project has Pak signing enabled.
• Loading pak files that have been created with a different engine version than your project might not load.

Code Modules:

• Runtime Module: "PakLoader"
• Classes: FPakLoader, UPakLoaderLibrary, UAsyncPakDownloader, FLogHelper

Technical Details​

Number of Blueprints: 0
Number of C++ Classes: 4
Network Replicated: No
Supported Development Platforms: Windows x64
Supported Target Build Platforms: Windows x64
Documentation: https://drive.google.com/file/d/1DeS0DlwLTue_jLNqjGHVHz6AHs89rDUM/view?usp=sharing
Example Project:
UE 4.22 Download
UE 4.23 Download
UE 4.24 Download
UE 4.25 Download
UE 4.26 Download
Forum Support: Link