You can use whatever program you want, it’s about communicating ideas and show what you have done...you can use Adobe programs for that, illustrator and indesign but if you don’t have access to those, use whatever you have, can be done in PowerPoint or even Excel it really doesn’t matter. Actually the cell structure of Excel can come in handy to build a good looking page, it has already a grid structure.

InDesign is recommended 

You can get by with a free version of Gimp. Watch a few YouTube vids and it will walk you through it 

good luck kid 

OLA got a good suggestion. InDesign or if you can't afford it, you could use Scribus. You can use GIMP but I would use it as a free alternative to Photoshop but Scribus and perhaps some others are good alternatives that might not cost as much. I heard of Affinity Publisher and Affinity Designer which you might be able to do what you want and export out as a PDF and only cost like $25 for each of them at this time. I have used Scribus in the past and its free. 

Affinity isn't too bad from what I can tell and you can get the version you want for Windows or MacOS and would not be stuck with a monthy subscription of like $20 or so a month.

If you have individual pages as PDF files you can use Adobe Acrobat and the combine files option (on my version it looks like a little green plus sign) select portfolio and then add the pages in, arrange them in order and it combines the files into a single portfolio PDF. The benefit with this approach is if you have many more pages than you can submit you can easily customize each portfolio to the specific application.

Hope this helps

Over and OUT

Peter N