LabelChop also has a free web tool for one-off label conversion:
https://labelchop.com/tools/a4-to-4x6-shipping-label-converter
Use it when you just need to convert an A4 shipping label PDF into a 4x6 / 100x150mm thermal label.
You only need to fix one or a few labels.
You want to test whether the PDF shape is the problem.
You are not ready to install the desktop app.
Use the LabelChop desktop app if you print labels regularly and want the process automated.
The desktop app watches your folder and prints labels automatically, so you do not need to upload and convert each PDF manually.
