If you have multiple PDF files that want to combine into a single PDF file, try with this PDFelement for Mac. This program will let you add as many PDF files you want to merge into one single PDF file.

youtu.be/CuQArPdsRxg

Moreover with this PDFelement:
Edit PDF text, images, watermark;
Extract data from PDF Form;
Perform OCR PDF files;
Protect PDF with password