Like Apple Books or Photos, Yomu is not a document based app that opens files in place or stores them in a folder. When you add a book, a copy is created and saved in your library. If you have enabled the Cloud Library (pro only), it will also be stored in a private database of your personal iCloud account.

This means that you only have to import books once, but you won’t be able to access them directly later. You still can save or export individual books and their annotations afterwards by tapping on the context action button (3 dots) next to a document in your library or long press (Options > Save/Export). It is also possible to save all original book files using Apple Shortcuts to create a backup to iCloud Drive using the Save Book shortcut.

Yomu is intended as a simple ebook reader. It’s recommended to only add books you are currently reading and keep your entire collection on some storage provider (e.g. Dropbox) or use a dedicated ebook management software like calibre to organise your library. In any case, it is important to keep a backup of all your original files.

This way, you can manage your books with a dedicated tool and are also not locked into one app. For example, you can then use Yomu together with any other app or reading device such as a Kindle.

The maximum size of files you can open depends on your device and the available memory or storage space. Most ebook formats are compressed files (like a zip). When you import or open a book, they need to be extracted and parsed to be displayed. This will take up more memory and space than the original file.

Devices like iPhone or iPad have memory limits, apps can become slow or are terminated by the operating system if they use too much memory. This is not a crash of the app, but the system’s watchdog that kills it. Another problem is that the operating system reserves disk space for critical updates. While it might look like you still have space left on your device, apps can’t read or write files anymore.

Available memory and disk space depend on the device type and model. It’s quite restricted for iPhones, while iPad or Macs can open larger files. As a rule of thumb, anything above 1GB is usually too much for devices to handle.

You can try to free up memory by closing other apps and restarting your device or increase the available disk space by removing items you no longer need. Refer to Apple’s support documentation Manage storage on iPhone for more details.

Another option is to optimize books by reducing their size. For example for comics, you could use CBconvert to convert images to the compressed AVIF format. Yomu is also primarily intended for text based formats (epub, mobi, kindle), you might be better off using a dedicated app for large pdf or comic books.

Location settings are a system feature that work in a somewhat non-intuitive way. As long as an app has not yet requested your permission, it will either not be listed in your device settings or it can’t be disabled. Settings only allow to change your preferences later.

The first time an app tries to access your location, it must ask for your permission. Apps won’t use your location until it has been requested and you grant permission.

In case you have not used this feature, you would need to first open a book and create an annotation or bookmark. This will show the corresponding location permission dialog: tap on “Don’t allow” and it should be disabled in settings.

Likewise, if you have already granted the location permission and changed your mind, you can update your preferences from Settings > Privacy > Location Services. Scroll down to find the app, tap the app and select an option.

For more details, see Apple Support:
Turn Location Services on or off
About privacy and Location Services

You can uninstall Yomu like any other app on your iPhone, iPad or Mac.

On your iPhone or iPad, touch and hold an app from the home screen until all apps start to jiggle. Tap the remove icon in the upper-left corner of the app and confirm deletion.

If you have enabled the Cloud Library, data will still be stored in your personal iCloud account. In case you want to completely remove everything, you will need to manually delete it from the device settings.

Caution: Deleting iCloud data containers from your device settings will delete it from your iCloud account on Apple’s servers. This action can not be undone and your data can’t be recovered:
Device Settings > Apple ID > iCloud > Manage Storage > Yomu: Delete Data

Mac Catalyst apps don’t have an uninstaller, you will need to manually delete it. Move the Yomu.app from the Applications folder to the trash. Local data will still be stored in the app’s container.

Open Finder or the Terminal to delete the app’s local data container:
/Users/Your-User/Library/Containers/Yomu

The Library folder is hidden on Mac by default. You can show it with Finder using:
Finder > Menu > Go > [hold option key ⌥] > Library