Single-page PDFs appear as an expanded document (View in Place) unless they are blank, in which case they appear as an icon. If you want the single-page PDF to appear just like multi-page PDFs, as an icon, then right-click on the single-page PDF icon and choose View as Icon.
Quit Apple Mail. You can tell Apple Mail to always iconify attached PDFs by launching the Terminal, copy the following, and pasting into the Terminal, followed by a return:
defaults write com.apple.mail DisableInlineAttachmentViewing -bool yes
The next time you launch Mail and attach a single-page PDF to the Mail compose window, it will be iconified, and the right-click option to VIew In Place will be disabled, though you can single-click it and press the space bar to view it as a Quick Look item.
All of this is for your benefit as it does not control how the recipients Mail application presents the attachments to them.