Hi @Sang,
Thank you for posting in the Lucid Community! An EDU free account is the same as the EDU premium. Although it says unlimited document creation, our free EDU upgrade only allows you to edit your 3 most recently edited documents.
If you make a copy of a view-only document (Navigate to your "My Documents" tab, right click over the document, select duplicate) the system will prompt you to rename your duplicated document, then it will be saved as the “most recently edited”, thus allowing you to make changes. You can do this for any documents that subsequently get pushed to view-only. You can also delete any old iterations of the document without compromising your edits.
Please let me know if you have any further questions!