I have a .vhdx file from an old virtual machine and I want to take a few documents out of it. How do I open it on Windows 10?

Windows can mount VHD and VHDX files as a drive.

Easy way: right click the file in File Explorer and choose Mount. It appears as a new drive letter.

Disk Management way:
1. Right click Start > Disk Management.
2. Choose Action > Attach VHD.
3. Browse to the file and click OK (tick Read-only if you only need to copy files).

Copy your files, then right click the new drive and choose Eject, or use Detach VHD in Disk Management.

If the virtual disk uses a file system that Windows can not read, like Linux ext4, it will show as unallocated. A BitLocker protected virtual disk asks for the password.

Mounted it and copied the files. Thanks.