Preview Cant Open Multiple PDF in same Window
You can open multiple PDFs in the same window, but the UI is different. Multiple PDFs will now open in separate tabs rather than the old way of segmenting them by scrolling.
Here is a link to the Apple support site if the following steps are not clear: https://support.apple.com/en-us/HT206998
To ensure all documents open in tabs follow these steps:
Big Sur
(Open) System Preferences &gt; (Select) General &gt; (Go down to 5th option) Prefer tabs: &gt; (Select from option menu^) Always
Older versions of macOS
(Open) System Preferences &gt; (Select) Dock &gt; (Go down to 5th option) Prefer open tabs when opening documents: &gt; (Select from option menu^) Always
Now, when you open multiple PDFs they will open within the same window, segmented by different tabs.
^From the option menu, you have the following options:

Always: When you open another window in your app, it automatically opens as a tab in your current window.
In Full Screen Only: When you open another window in your app, it opens as a tab only if you're using the app in full screen. Otherwise, it opens as a new window.
Manually: Manually choose when to use tabs, whether or not you're using the app in full screen.