Sorry, I forgot to specify. I tried some of their options but this one i think worked for me:
I solved the issue for someone else and saw that it had something to do with the file's property. Here is how you can solve it.
Open the pdf in Word doc. Microsoft Word and save it under a different name since it will be read-only.
After saving the file, save it as pdf.
What is the file size for you? Mine was at 776KB but yet it was not working. So I did open it in WORD then saved it as a PDF. Then uploaded it.
You can also try this option:
We were having the same issue when even we reduced the file size to 365kb. I sent the original document to my email. Instead of downloading, open print option and "save as pdf" instead of "Microsoft print to pdf." That reduced the file size to 236kb, which i was able to attach and submit without any issue. For reference, the original file size was 443kb.
Be sure to keep the original copy saved somewhere and create copies to follow these options..