Simply go into the app where your file is, and look for an "Open" or "Share" button. The buttons usually look something like this:

Next, you will see a screen like this. Look for the option to "Save to Files", or open in your Files app. You may also see the option to open the file directly in the Converter app. You can choose that as well.

Then go back to the converter app, and choose the option to Open from your Files app. Choose the file you just saved.