If you have turned in an assignment that shows up blank after you turn it in, you most likely have not followed the process to writing on a PDF correctly.
If you are working with a PC you must download the assignment first and immediately save it to your computer. If you do not save it to your computer you will not be able to capture the answers you type into the assignment. Once it is saved to your computer you can go ahead and begin to type into the assignment. Once you have completed the assignment you MUST save it again to your computer to lock in the text you have put in there. Then go to the website and submit the assignment again and it should work at this point.
This process is a bit different than the process for PC. Once you download the file you will right click on your mouse or track pad and it will bring up an option to "Get Info" on the file. Once you click on that option and all of the information comes up you must change the file from an adobe file to a "Preview" file. Once it is switched to a preview file you MUST save the assignment to your computer as that will allow you to type in on the PDF file and eventually lock in your answers. Once you are done with the assignment make sure you save it again. Then go to your profile again and go to the "Homework" section of your profile and submit the file.