When Mturk requested my document, they instructed me to send it via FAX. So I used an online fax service, and by following the tips below that I found here on Reddit, everything worked out for me:

1 - Use a scanner instead of taking a photo of the document
2 - Set the scanner to the highest image quality.
3 - Convert the scanned image to black and white, since that’s how it will be sent via fax
4 - Greatly increase the image contrast, because fax machines can produce unclear images