Callback not firing in index.js for files greater than 7MB

So, I was using this to generate images for my pdf files. I tried testing it for small files and it worked. 

However, when I tried it for files greater than 7MB, the png is getting generated but the callback function is not getting fired. Is this a common problem or is this a unique problem to this only?

Also, what could be a good solution for the same?