The bottom of the file menu displays a list of recently accessed files. You can click on these to open them without having to interact with a file chooser dialogue.
The open file option can fail if too many files are currently open (active). If this occurs, you may close, save, or export one of the other open files, and then try again.
ACORNS remembers the folder that housed the file that was opened last. This eliminates unnecessary traversing across system folders.