Failed to open PDF certificate

When the Student tries to download the Certificate’s PDF and open, it gives the “38d73c0a48be3035e01e6d593748.pdf” could not be opened.

https://cdn.livechat-static.com/api/file/v2/lc/att-old/8801096/97f9238c737397cf833499b8b7c7baeb/Screen%20Shot%202018-10-08%20at%2010.57.05%20AM.png

I tried to download some PDF from /wp-content/uploads/pdf-cache and it opened ok, can you help me?