Error 403 when creating a button linking to pdf file

Hello,
I experienced a problem on one of the customer’s site recently. I tried to add a new post with Page Builder and two widgets: Editor + Button.
Button was supposed to link to pdf file, but when I try do publish the new post I get the error 403.

[Thu Jun 06 13:54:46.155480 2019] [:error] [pid 28:tid 140204199356160] [client 194.181.252.78:52529] [client 194.181.252.78] ModSecurity: Access denied with code 403 (phase 2). Pattern match "http://[a-zA-Z0-9._]{1,}?/.{0,}?\\.pdf\\b[^\\n\\r]{0,}#" at ARGS:panels_data. [file "/usr/share/waf/09_Global_Other.conf"] [line "21"] [id "211050"] [rev "1"] [msg "COMODO WAF: Universal PDF XSS URL Detected.||||2"] [severity "CRITICAL"] [tag "CWAF"] [tag "Other"] [hostname "www.mydomain"] [uri "/wp-admin/admin-ajax.php"] [unique_id "XPj-Bqv34OrCqnvJli15GwAAAAU"], referer: http://mydomain/wp-admin/post-new.php

It seems that server recognizes button as a potential threat, but when I create normal url leading to pdf everything works just fine.
I saw couple of threads refering to 403 error issue, but none of them seemed similar to mine.