Files are opened in individual tabs.
Create a new file tab by clicking the + icon on the tab bar. From there you can choose a file type or pick from the recent files list.
Bring up the file open window by pressing
This will bring up a list of recent files and files from the current active site.
You can type to filter through this list and find the relevant file.
Tabs can be re-ordered by dragging them.
Tabs can even be dragged from one panel to another e.g. from the file panel to the preview panel.
This allows you to have one file alongside another file or a vertical SSH panel.
You can drag local files from your desktop onto the file tab bar to open them.
Right clicking the tab will give you options to "Close All Tabs" and "Close Other Tabs".
Tabs can be set to re-open on start-up.
Gear Icon -> Preferences -> Files -> Restore tabs on startup
You can bookmark a group of files for faster opening.
Right click tab -> Bookmark all files..
Reveal in file tree
You can locate the currently opened file in the file tree.
Right click tab -> Reveal in file tree
Some other useful and self-explanatory preferences include:
Edit -> Preferences -> Files -> Strip whitespace on save
Edit -> Preferences -> Files -> Save with minified
Edit -> Preferences -> Files -> Compile LESS files on save