Because there are many different PDF creators floating around the internet and no industry standard, you'll get a PDF document once in a while that will cause some problems.
If you're on a Mac, open the PDF in preview and click File, then click Duplicate. From there, save the duplicate file to a different location. Be sure to upload the copy - not the original file!
If you're using a PC, you can use the program PrimoPDF to basically do the same thing.
Once you've downloaded and installed PrimoPDF, open the document and select Print with PrimoPDF, and then click Print.
This will actually save the document as a PDF.
Once you've saved the new document, try uploading it to HelloSign again.
If it's still not working, feel free to email the document to us at email@example.com. Please be sure to include all the steps you took and what happened, and we'll investigate.