We are being given a PDF with some unredacted text. We can easly observe that the redaction has been done poorly, as we can copy the content of the document (CTRL-A).
However we realize that there is no secret data hidden in the text. So we prooced by checking the metadata.
By looking at the properties of the file we can easly observe that the 'Auhtors' field contains a weird string. This sting ends with '=' so we proceed by tring base64 decoding.
It turns out that this string was the base64 encoding of the flag.