Both Windows and ChromeOS have a built-in tool for unzipping files. So, they shouldn't need a third-party tool to extract files from the zip archive. Ensure the file is downloaded to their computer and they are following the right instructions.
To open the zip file on Windows or ChromeOS, one needs to right-click the zip folder and select "Extract All." If they used the same steps but still couldn't open the zip folder, consider compressing the PDF files using a third-party compressor and sending the zipped file to them again.
The file compressor in iBoysoft MagicMenu is a good choice. It also allows you to right-click the selected PDF files and choose Compress to create a zip file as the built-in Archive Utility. However, it provides more advanced features like password protection and customized compression levels.
Test it out by clicking the button below!