You want a.pdf scanner app or photo scanner apps, which scans multiple documents and make them a .pdf file? I can tell you the second one. Camscanner is a really good app, which scans multiple photos and then convert then into single pdf file. It has both free and paid version. In the free version the app creates a very small watermark ‘ scanned by Camscanner’. That is not a issue for a regular user.
If you have never used ImageMagick and don't know how to use it, check out this video tutorial. If you want to create a GUI from scratch, here is an excellent tutorial. If you are new to ImageMagick, I have written another introductory tutorial. Update: It seems that ImageMagick is no longer free. See this note. It is free. I use it daily to convert my images into PDF files. One of the features I love a lot is the ability to resize and crop images in a single batch. If you are a Windows user, and you want to add a tool to your command line, here is a great tutorial and blog entry. There is no GUI. It works almost like this: Useful tips if you are a Windows user. You can convert between various file formats, even including other image formats (like the “image” of a web page). One of the best features.