How to make quick collage from PDFs (each is event photo smaller than paper size)?

Using Adobe Express Web, I need help to collage photos from many PDFs containing images only?

These photos are recordings of a training event.

Currently, I am 

1. On computer
2. create Interim Folder
3. open a PDF
4. export image
5. crop image to rid white space due to paper size larger than photo 
6. repeat above for each PDF
7. open Adobe Express
8. How to Collage?