There are various reasons due to which inbound media messages can fail. In the case of PDF files, the usual reason for failure is thumbnail validation.
For an Inbound message consisting of a PDF file, Facebook will validate the thumbnail size of the PDF file. Thumbnail sizes of only up to 64kb are allowed. If the limit is exceeded, the inbound message will not be delivered to your WhatsApp Business API, as the pdf fails at the receiver side (business) due to thumbnail validation. Additionally, in such a case, no inbound event is received to explain the reason. Please make sure that the file sent by the user, complies with WhatsApp's requirement.
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