I had a chance to look at it today. Your post gave me an idea what this issue might be. I use a WordPress plugin for security and part of it I have set up to block hotlinking.
# CUSTOM CODE BOTTOM HOTLINKING/FORBID COMMENT SPAMMERS/BLOCK BOTS/BLOCK IP/REDIRECT CODE - Your Custom htaccess code will be created here with AutoMagic
# BLOCK HOTLINKING TO IMAGES
SetEnvIfNoCase Referer "^(http|https)://.*\.firstgenfirebird\.org$" whitelist
SetEnvIfNoCase Referer "^(http|https)://.*firstgenfirebird.*" whitelist
SetEnvIfNoCase Referer "^(http|https)://.*oakmediacreations.*" whitelist
SetEnvIfNoCase Referer "^(http|https)://.*firstgenerationfirebird.*" whitelist
SetEnvIfNoCase Referer "^(http|https)://.*\.google\.com$" whitelist
SetEnvIfNoCase Referer "^(http|https)://.*\.yahoo\.com$" whitelist
SetEnvIfNoCase Referer "^(http|https)://.*\.bing\.com$" whitelist
<FilesMatch "\.(gif|jpg|jpeg|png|bmp|pdf)$">
Order Allow,Deny
Allow from env=whitelist
# Add Your Server IP Address
Allow from xxx.xxx.xxx.xxx
</FilesMatch>
I removed the pdf part and it resolved this issue.
Thank you - Geoff
