Since I updated to macOS Sierra (10.12.2) PDF attachment are shown with sizes of 0 KB and a preview is not embedded. Resetting Mail and even re-installing the whole macOS system (factory default, no recovery) did not help.
The only way to read the attachments is to view the e-mail raw data (⌥+⌘+u) and then store it into an *.eml file. The created eml file can then be opened with Apple Mail via ⌘+o and the attachment is readable.
What am I missing? My inbox is IMAP not POP3 and my provider is posteo.de.