One of the worst things about Acrobat is it can do so much more than any other app, so there is a lot more to learn. Portfolios, for example, are a really useful way to combine all sorts of files into a PDF "wrapper", so they stay the original files. It's also usable for example, to combine secure documents. You don't give a lot of details so let's make sure we are on the same page. A portfolio says it's a portfolio, and tells you that you have to open it in Acrobat, which is all true. When I make one, it looks like this in Preview.

Now, there are LOTS of ways to combine files. I'm assuming you are using File > Creat > Combine files into a single PDF. If so, you or someone may have turned on the portfolio option (it's switched off here). Click the gear icon to change it.
