My guess is that you mistake Arrays for Collections.
They are 2 different classes in vb.net.
Your MergePDF activity needs an array while your AddToCollection activity needs a collection.
From there comes your mismatch.
What I did is :
1 - I created a new variable that will iterate through your foreach, counter
2 - I renamed your collection_raab_files variable into array_raab_files for the semantic (please keep in mind that arrays =/= collections)
3 - Each time you add an item into your array, increment counter by one
Uipath binders maken facturen.xaml (21.3 KB)
Tell me if this works