Can I open documents in tabs instead of windows?
Opening in Tabs
In the most recent versions of PDFpen/PDFpenPro, you can open documents in tabs instead of a new window.
To do so, choose
View > Show Tab Bar.
To make tabs your default there is a setting in System Preferences that should help.
Open System Preferences -> Dock and change the value next to 'Prefer tabs when opening documents' to 'Always'.
Now close PDFpen and reopen it.
New documents should open in a new tab in PDFpen (vs. a new window) going forward.