Since you said you want to upload a PDF, I'm assuming you want to upload files to the Shopify Admin in the files section, instead of uploading an image or file to be added to a product page. Uploading a file to the Shopify Admin is a poorly documented and painful 3-step process. I recently figured it out myself and will gladly break it down for you. BTW you might not be able to pull this off using postman- I'm not sure how long the upload URLs last.
First: Prepare the file. Since you are attempting to use postman, you just need the file to be on your local machine or uploaded to postman. Normally you would download the file to your node app server and read it with the fs package.
1. Call the stagedUploadsCreate mutation. You do not feed the image through in this step, all you are doing is telling Shopify to give you a temporary upload URL for your file. You will need both the url and resourceUrl, as well as the parameters that it returns.
2. Post your file to the returned url (not the resourceUrl) with the returned parameters using FormData. The file is added to the formData.
3. Call the fileCreate mutation, using the resourceUrl.
You can successfully upload any file type using this method, you just have to change your file name extensions, mime type, and contentType. Here is a much more in-depth explanation that helped me with a full code snippet for NodeJs to get you up and running in no time: Full process for uploading files to the Files API - Shopify Community
If you don't use NodeJs, try copy/pasting the code from that post into your favorite AI tool and ask it to convert it. Hope this helps!
