One of the reasons that blank pdf files get uploaded is because the pdf file was edited in the browser (e.g., edited in Chrome).
To prevent this scenario, the file must be downloaded and edited on the laptop.
In the Chrome browser:
Click the three vertical dots in the upper right hand corner of a Chrome window to reveal the browser's menu options.
Click "Settings".
Scroll down the Settings page to the "Privacy and Security" section.
Click on "Site Settings"
Scroll down to the "Additional Content Settings" and click the down arrow to expand that section, if required.
Click once on "PDF Documents."
Slide the toggle button to the right to turn on "Download PDF files instead of automatically opening them in Chrome".
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After updating the settings as shown above, PDF downloads will be placed on the laptop going forward. Your child will edit the document on the laptop, save it, and upload it from the laptop into the assignment for submission.