I use a reader called APV. This can be set to stay in landscape with the rest of the Android interface in portrait.
Reading academic PDFs is OK in landscape if the text is one column across a normal A4 page width. A giant textbook presents problems. You have to read down one column then scroll back up, and the response is not fast with a very big file. Pictures may well be hard to see across the full width.
I don't feel the need for multitouch as there is a zoom button (autohides itself) available all the time. If e.g. the margins change, you can tap "fit to width" then tap the zoom to adjust. Only takes a second. It is not possible to switch between documents with this software, but you can just open a different PDF and it will remember your page, so it is effectively the same as if both were open at once. It is three presses (menu, open, choose file).
Basically it is not a problem so long as the PDFs are all text and in one column on a normal size page.