The short answer: nothing is lost. Both compression and uploading resume exactly where they stopped. You can close the app whenever you like.
During compression
Closing the app mid-compression is safe. Files already converted stay in the output folder, and the next time you run the same job the app skips them and carries on with the rest.
If you press Cancel after a lot of files have been done, the app confirms first — finished files stay put, but the remainder will not be processed until you run it again.
To force a complete redo, there is a Force re-process every file (ignore resume cache) option under Advanced settings. You will rarely want it.
During upload
The upload view has Pause, Resume, and Cancel buttons, and it says plainly:
Safe to close the app — the upload resumes where it left off.
That covers closing the app, shutting the laptop, losing Wi-Fi, or a power cut. Files are sent in pieces and the app tracks which ones have landed, so restarting never re-sends what already arrived.
When you reopen the app on a batch that was mid-upload, the button reads Resume upload rather than starting a new one.
If files failed
The upload view shows a failed count alongside the successful one, with a Retry failed button. Retrying only re-sends the failures.
If more than 5% of the batch is still missing, the app will not let you finish the batch without retrying. Under that threshold, you can choose to finish without them.
If you cancel an upload entirely
The batch stays on your account in an unfinished state. You can pick it up later — either from the app, which offers to resume it, or from the website under Projects, where the project will show a Reupload or Resume payment button.
See Resume or re-upload a batch.
What is not resumable
The processing that happens after upload runs on our servers, not on your computer, so it is not affected by anything you do locally. Once the app says the upload is finished, you can close it, turn the computer off, and go home. The report is built either way.