This has been discussed multiple times over the years, both related to 
Horde and more generally how Apple mail is horrible how it structures 
the Mime parts in messages like this.

The quick answer is that mime structure says that there are two 
alternative ways of displaying the same content, a "text" 
representation and a "mixed" representation. IMP is displaying the 
text representation (since it's not possible/practical to render the 
pdf in-line, sandwiched between the other two text parts).   
Multipart/alternative parts are not where attachments belong.

You can see the other parts by either using the "show all parts" (as 
you discovered) or by setting your "message viewing" prefs to "show 
all parts" in the summary.

When an email with attachment(s) gets forwarded via Apple Mail the 
resulting mail seems to break the attachment-identifying process in 
IMP, so they aren't visible.

They can be shown (and downloaded) via the "show all parts"-function, 
but not with the "show attachments"-setting in use.

Within MUAs like Thunderbird, Outlook and The Bat the mails looks fine.

MIME-Structure produced by Apple Mail:
Alternative
* Text
* Mixed
** Text
** PDF-Attachment
** Text

The Mixed-Part isn't visible in any way without the "all-parts"-Option on.

Attached there's a raw sample mail.