FormMaster,
The PlanetPress Image does not concatenate PDF files. It creates images.
If you want to contatenate PDF files you can do a few things. First, in PlanetPress Workflow 7.1 there is a new input plugin called "Concatenate PDF" which can work by taking all of the PDFs in a folder with a certain mask, and put them all in a file.
Second, you can use a "Send to Folder" output with the "concatenate" option checked to do it (this, however, requires Office or Production, it does not work in Watch).
Lastly, you could use a PlanetPress Design document to display all your PDFs using showpage() commands in a runpage with loops - but that's not an easy solution.
Hope this helps,
Eric.