tl;dr: This program did not work for me, and I am giving up with it. Below I explain why. Please do not think I am being harsh, but I just had a terrible (and very unusual for me, I never had so many headaches with a program before) user experience.
In my life I have never felt so overwhelmed by a software like this. So much indeed that I decided to register here to post my problem even though I only had to keep using my old good friend online pdf converter and call it a day.
One problem is that every document I create is vertically flipped (180°). It does not matter if it is some pdfs that I want to merge or some PNGs. The only instance it worked was because I flipped every individual PNG on windows before feeding the files into the program. And even that time It was not satisfactory because the resulting document "cut" the borders of my original PNGs. I guess I needed to change the output page size to be larger, but could not find where to do it.
The funny thing is that when registering to post in this forum, the "Your first notification! Select it to begin" bugged out and I could not unclick it, blinking in and out of the page with each clicking. It solved after refreshing the page, but I was not impressed.
I lost at least a couple hours trying to find why this program was not working for me, but I am just giving up now. The online pdf converter just works flawless, I just have to wait a bit for upload and download as my bandwith is not that good. I do not understand: Is this program too difficult to use for common users like me? I tried this software because it was recommended in the website of the institution where I need to submit some documents, and I am giving away this feedback because it is the first time I felt like this with a particular program.