Sometimes there are conflicts with the file and the file viewer, for example, the signing boxes do not open, you lose all the signs after signing or the PDF file does not open correctly. If this happens, please try with a different PDF viewer and make sure that you are running the latest version.
Bear in mind that if the document has several pages, it might take a while to upload, also the first pages to upload are the ones with a signing box in them.
If the problem still persist, contact us and attach the file and a screenshot. The more information we have, the faster and better we can help you.