# read the textfile as single ASCII string
$hex = [System.IO.File]::ReadAllText('D:\Test\blah.hex', [System.Text.Encoding]::ASCII)

# convert to bytes and write these as new binary file
[System.IO.File]::WriteAllBytes('D:\Test\blahblah.exe', ($hex -split '(.{2})' -ne '' -replace '^', '0X'))