I have the limit for attachment size set at 0 (zero) but some pdf attachments will not 'attach."
Tried to attach a pdf file of 62 MB. It appeared to upload successfully (although slow) but the attachment tab never cycled to allow me to place it inline.
I attached a pdf of 800kb and it worked fine.
One of my forum member PM'd me with a similar problem--when he tried to attach a pdf file he got an HTTP(?) error.
In the High Desert of Central Oregon