The posting of a PDF file to a website in and of itself doesn't change the content of the PDF file. If the checkboxes appeared in the PDF file before you posted the file, if you download the PDF file from the website and open the PDF file in Adobe Reader or Acrobat, those checkboxes should still appear.
That having been said, if you display the PDF file in the browser window, what you actually see depends upon what exactly is displaying the PDF file. For example, if your web browser is Chrome, Firefox or Microsoft Edge (Windows 10), the display of PDF within the browser window is provided by the browser which in none of these cases is a full PDF renderer. Similarly, with the MacOS Safari browser, unless you install Adobe Reader or Acrobat and have properly configured the browser to use same, the PDF will be displayed by a built-in renderer (i.e. Preview) which is also not a full function PDF renderer. Similar issues occur on iPhones and Android devices if attempting to view PDF with non-Adobe PDF renderers.
You didn't indicate how the checkboxes were created. If they were created as squares with very thin lines, the lines just may be too thin to properly display for some PDF renderers. The workarounds would be either (1) use thicker lines for the outline of the checkboxes or (2) the preferable solution of using a character from a font for this purpose. In the latter case, many fonts have what is known as a ballot box character. You can readily find such a character in the Microsoft Wingdings font.
- Dov