how to open more documents

I need to have articles in front of me so I can remember to get back to them while also opening which ever texts I would need at any given present moment. Unlike the default app on the mac, Adobe has this very curious feature where they will cap the number of documents you can have opened. How can I either  1- circumvent the problem or 2- if that doesn't work, not have Adobe open my pdf's by default but also enable "Aperçu" ("Outlook" in English? Whatever the Macbook gives you - "Capture"? - to read images and pdfs) to respond when I try to open a document (without having to uninstall Adobe)