PDF app for creating and merging

Hello All:
I have recently purchased some new Apple products. They have emailed me all my receipts in .pdf files. I have several of them. I am looking for a good PDF app for iOS that I can use to merge all of these individual files into one file. I don’t think I can afford an Adobe subscription. Anyone have any good suggestions on an app for iOS or another means of combining or merging several pdf files into one file?
Thanks,
Dave Matters ...