You can set this option in the contact form which is the object. When you create a form, select from the list e.g. name, phone, message etc. "Attach file", then anyone who uses the form will be able to send the file. You can also do only one section in the form, e.g. "Attach file".

Hi Chia, 

thank you for getting in touch. 

If you need a user to be able to upload images or texts autonomously without using the software, you could try using the Dynamic Content Object which is available for the Pro edition of the software. If you want to have a look, you can try the demo version here: https://www.websitex5.com/download/v2019.3/wsx5_pro_demo.exe

Unfortunately, this Object is not suited for uploading PDF files as well. Still, please let me know if you have any doubts, I will be happy to help you. 

Thanks! Kind regards.