When saving or exporting forms that contain checkboxes from Microsoft Excel (for example, financial forms) to PDF, the checkboxes may appear as black boxes in the resulting PDF. This will only happen if you are saving or exporting the form using Microsoft Excel's built-in PDF capability - it will not happen if you use a 3rd party PDF creator.
This problem is caused when a certain export option is enabled - specifically, the option "ISO 19005-1 compliant (PDF/A)."
When you save or export a form to PDF, you can click on the "Options..." button prior to saving or exporting to select specific export options. One of these options will be the "ISO 19005-1 compliant (PDF/A)" option. If you are experiencing this problem, make sure that this option is not checked.
Once you have un-checked this option, you can save or export your form to PDF and the checkboxes will appear normally. You do not need to un-check this option every time; Excel will remember your choice (unless you change it later on).