Unable to open attachment in Roundcube: "Error: Failed to load PDF document."

Symptoms

• The attachment of an e-mail can not be opened with Roundcube and other mail clients (Thunderbird, Outlook, etc.):

  PLESK_ERROR: Error:
  Failed to load PDF document.

• On the sender's side under  Roundcube > Settings > Filters a rule is present with Send message copy to: in execute the following actions

• The following versions of pigeonhole and dovecot are installed on the server:

  • For Redhat/CentOS based OS:

    # rpm -qa | grep dovecot
    (...)
    plesk-dovecot-2.3.0.1-centos7.18031217.x86_64
    plesk-dovecot-pigeonhole-0.5.0.1-centos7.18031217.x86_64`

  • For Debian/Ubuntu based OS:

    # dpkg-query --list | grep dovecot
    ii plesk-dovecot 2.3.0.1-debian9.0.18031217 amd64 Dovecot v2.3.0.1 secure IMAP and POP3 server
    (...)
    ii plesk-dovecot-pigeonhole 0.5.0.1-debian9.0.18031217 amd64 Sieve filters support and secure ManageSieve server for Dovecot v0.5.0.1

Cause

This is Plesk bug with ID #PPPM-8470, which is planned to be fixed in future Plesk updates. 

Resolution

As a workaround send the e-mail with attachment to the recipient directly.