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

Applicable to:

#PPP-36408 "If the configured Sieve rules copies an email with a PDF attachment to another mailbox on the same server, the PDF file is no longer corrupted."
Fixed in:
- Plesk Onyx 17.9 Preview 6 25 September 2018 (Linux)
#PPPM-8470 "Large attachments are broken (dovecot, sieve) (as PPP-36408)"
Fixed in:
- Plesk Onyx 17.9 Preview 6 25 September 2018 (Linux)
- Plesk Obsidian Release Candidate 1.0 04 June 2019

Please consider updating your server:

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

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

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