You can transfer files from other apps to PrintCentral.

You can import files from the Apple Files app. Open the Files screen in the left side panel of PrintCentral. Tap the "+" button and then tap on "Import". In the Places screen choose Files App to choose file.

For other apps:

1) You can share files with PrintCentral using the Files app. PrintCentral will appear in the Files app in the locations section. Tap to open the button called "On my iPad" or "On my iPhone". Then you can see the PrintCentral shared folder. You can save files from other apps to this folder. You can also view files from this folder in other apps if those apps support this feature.

2) Use "Open in" or "Share" from your other app and select PrintCentral. This option should be available in most apps when selecting files and using an action menu

3) - Multitasking on supported iPads
Open the Files (or Web Browser) screen inside PrintCentral
Slide up from the bottom of the iPad screen to show the app bar. Tap and drag on another app (which supports drag eg. Photos, Safari, Files) to the right of the screen to create a split screen.
Tap and hold on document, photo or other content to drag to PrintCentral to save or print.

4) Use "iCloud Drive" from your other app and tap on "EuroSmartz Apps". This will save the documents from the other app in the iCloud folder inside PrintCentral

5) Use a Cloud Server to share the file with other apps