Add PDF pages from different folders into one pdf in the desired sequence

I was trying to join different PDF files into one in the desired sequence, but in the activity it gives only option to input a string array and output as one pdf location, what i was trying to do is combine different pdf files into one but there is no option to join from different locations say like different folders where files are placed and finally get the single pdf in intended format like first pdf page will be introduction then pdf pages contain some content then finally a closing pdf page. When i put all these different pages in one folder we can guarantee the sequence in which they are joined is what we intended.