# Blank Paper
We're given what is supposed to be a PDF document, but can't be opened as it is missing its magic number, or signature, which denotes that it is indeed a PDF file. This is evident when we open up the file in a hex editor such as Bless.
![](https://raw.githubusercontent.com/shawnduong/ctf-writeups/master/2019-ANGSTROM/images/blank-paper-1.png)
The first four bytes are missing. PDF files have a signature of `25 50 44 46 2d`.
You can see a list of file signatures here: [Wikipedia](https://en.wikipedia.org/wiki/List_of_file_signatures)
To fix this, we can simply just add the signature.
![](https://raw.githubusercontent.com/shawnduong/ctf-writeups/master/2019-ANGSTROM/images/blank-paper-2.png)