PDF Expert for Mac supports several ways to open PDF files.

• Drag and drop. On the ‘New tab’ screen, drag-n-drop any PDF file from Finder to a highlighted area.
• Open directly. On a ‘New tab’ screen, press ‘Select file…’ and choose a document from the file selection window.
• Drag the file to the Dock icon. When PDF Expert is in your Dock, drag-n-drop the PDF file on the PDF Expert icon and the file will be opened in a new tab.
• Click 'File' in the upper menu and select “Open” or use the “Cmd+O” shortcut.
• Double-click any PDF file to open it if PDF Expert is set as a default PDF reader.
• There is also a setting to open a new window instead of a new tab when opening a new document. For this, you can simply drag the tab with the file outside the PDF Expert viewer and it will get opened as a new window. 

You may choose one option that suits your workflow best or use all of them.