PDF Expert for iPad has been updated to version 4.7, adding iCloud sync, PDF merging, and a host of fixes and improvements. People have previously been able to sync their documents using other methods, such as Dropbox, but iCloud sync is native to iOS and requires not extra accounts or setup. The other significant update to PDF Expert comes in the form of merged PDFs.
To sync documents to iCloud, just move documents into PDF Expert's iCloud folder. PDF Expert also shares an iCloud folder with another Readdle app, Documents, letting the two apps share files simply by adding them to the iCloud section of either app.
To combine two or more exisiting PDFs into a single document, tap the Edit button in the documents list, select the PDFs that you want to merge, then hit the new Merge button. You now have a brand new, merged document.
The update for PDF Expert for iPad is available now on the App Store, while an update for PDF Expert for iPhone is coming at a later date.