Can I open a file in plaintext (already decrypt) but the cipher doesn't change? I want to create an app that encrypts a file and saves it to the Dropbox. Thus, I open and edit this file, I want the file is already decrypted, but the encrypted file that stored in Dropbox still in cipher text. Whenever I update/edit this file, encrypted file that stored in Dropbox also update. Is this possible?
Yes, it's possible.

Let's assume that you have a text. It is possible for decrypt the text content from the ciphertext when opening the file. The content can be present in the memory without being written to disk. When you're done, it can be saved, which will encrypt the plaintext and write it to a file.

What you need to do:


Figure out the file format (this is probably symmetric, so don't forget random IVs and MACs/authentication tags).
Write either a standalone program, or a plugin for an existing program (e.g. LibreOffice). In particular


User interface,
file opening, parsing and decryption,
data serialization and encryption.