Hi,
any chance to provide some debug info? [1]

[1] http://www.claws-mail.org/faq/index.php/Debugging_Claws

As it turned out, the crash is dependent on the very specific PDF. Unfortunately, I can't attach that file cause there are personal information in it that I don't want to share.
Maybe this document has no content at leads the plugin to crash?

Definitely, there's some kind of check missing on that side, so probably
an null value of a field in the documentation info might cause to free some memory that has never been allocated.
Some debug info would clear all doubts though.

Output when claws-mail is called from a terminal in ArchLinux. I guess the last line comes when the button is clicked.

++ STATS ++ INC 0 0
claws.c:101:Starting Claws Mail version Claws Mail 3.8.1
main.c:858:runtime GTK+ 2.24.12 / GLib 2.32.4
main.c:866:buildtime GTK+ 2.24.10 / GLib 2.32.3
main.c:875:Compiled-in features:
main.c:880: compface
main.c:886: aspell
main.c:892: gnutls
main.c:898: ipv6
main.c:904: iconv
main.c:910: jpilot
main.c:916: ldap
main.c:922: libetpan 1.0
main.c:928: libsm
prefs_gtk.c:1037:new file '/home/raphael/.claws-mail/clawsrc'
prefs_gtk.c:996:new section 'Common'
prefs_gtk.c:996:new section 'Plugins_GTK2'
prefs_gtk.c:996:new section 'Plugins_Common'
prefs_gtk.c:996:new section 'fancy'
prefs_gtk.c:996:new section 'NotificationPlugin'
prefs_gtk.c:996:new section 'TrayIcon'
prefs_gtk.c:996:new section 'Bogofilter'
prefs_gtk.c:996:new section 'AttachWarner'
prefs_gtk.c:996:new section 'AddressKeeper'
prefs_gtk.c:996:new section 'AttRemover'
prefs_gtk.c:1037:new file '/home/raphael/.claws-mail/folderitemrc'
prefs_gtk.c:996:new section '#imap/web.de/Projects'
prefs_gtk.c:996:new section '#imap/web.de/Projekte'
prefs_gtk.c:996:new section '#imap/web.de/Rechtliches'
prefs_gtk.c:1037:new file '/home/raphael/.claws-mail/accountrc'
prefs_gtk.c:996:new section 'Account: 1'
prefs_gtk.c:996:new section 'Account: 2'
prefs_gtk.c:85:Reading configuration...
claws-mail: Fatal IO error 11 (Resource temporarily unavailable) on X server :0.0.

can you type:

bt full after it hangs?

Created attachment 1166 [details]
gdb backtrace

Hi, 
thanks for the backtrace, it doesn't seem to be a bug with the plugin or claws mail itself but more a GTK+ one.
I can't investigate further because I don't have a pdf to test, I'm sorry.
If you can, try to edit such pdf and delete the sensible data so that I can
see if I can reproduce the bug.

Another system, it seems to be the same crash.

Claws Mail version 3.8.1
GTK+ version 2.24.13 / GLib 2.32.4
Locale: de_DE.utf8 (charset: UTF-8)
Features: IPv6 iconv compface GnuTLS LDAP JPilot GNU/aspell libetpan libSM
Operating system: Linux 3.0.43-1-lts (x86_64)
C Library: GNU libc 2.16

Is it possible for you to try with a version of GTK+ <= 2.24.10?

I'll send you a file by e-mail (size ~1MB, 10 pages) that crashes on the system mentioned in comment #8.

Thank you, it helped to find out the issue.