Batch file rename using PowerShell on Windows: add filename prefix, change file name to all lowercase, and so on.

batch change filename to lowercase.ps1

<# Batch change all PDF file names to lowercase letters within a specific directory #>
dir C:\PATH\TO\FOLDER\*.pdf | Rename-Item -NewName {$_.Name.ToLower()}

batch change filenames.ps1

<# Batch add prefix to all JPG-images in a specific directory #>
dir C:\PATH\TO\FOLDER\*.jpg | Rename-Item -NewName {"prefix_" + $_.Name}