Combing PDF

I hav just bought a premium version of Adobe Acrobat for my iPhone 11. One of the primary reasons for upgrading is to to be able to combine .pdf documents. I have had success with  simple combining of two documents but have not been able to have any success with more than two docs. I now have docs 'converting in progress' going on for two days and cannot delete. So I now have two problems. I am a very disappointed having just paid for Adobe. Does anyone have any tips? The problem is mirrored on my other device -iPad Pro.