On macOS you can merge PDFs together using the built-in Preview tool. You can merge PDFs on iOS as well, using the built-in Files app.
Merge PDFs on iOS
- Open Files.
- Navigate to the folder with the PDFs you want to merge.
- In the upper-right corner, tap Select. Tap each PDF in the order you want them to appear as a merged PDF.
- On iPadOS, tap More in the lower-right corner. On iOS, tap the three dots icon.
- Then, tap Create PDF in the menu that appears.
A new PDF will be created that combines the individual PDFs that you selected.