There are several ways to transfer files from another app so you can convert them into a PDF.

From inside other apps
You can use the Action/Share menu inside other apps to choose the PrintCentral Pro print icon. If you do not see the print icon, scroll to the bottom of the screen and tap on "More/..." to choose it.
Tap on "Choose" in the PrintCentral Pro print screen and then choose the "Convert to PDF" printer to save the item as a PDF. Tap here (View) for details.

Use "Open in" from your other app and select PrintCentral Pro
PrintCentral Pro should open and display the print screen.
Tap "Choose" in the Print screen and then select the "Convert to PDF" option and Tap Save.

Share via iCloud/Dropbox/Box.net
If the other app supports Cloud Servers you can also use that to transfer files
In the other app save your file to your preferred cloud server
Open PrintCentral Pro and go to the Cloud Server screen from the Files screen
Select your Cloud storage service or tap the + to set it up
Select the files and tap the Print button. Then tap "Choose" in the Print screen and then select the "Convert to PDF" option and tap Save.