First of all - welcome to BC !
Is this all you have ? Or do you have access to the original notebook ? If you have access, I would suggest the best thing to do is to re-scan the whole thing and make sure to do it in gray scale.
I just downloaded the image you listed above and using reasonably sophisticated image editing software tried selecting the text by colour, in the hope of then being able to paste it onto a white background. This didn't work because either it picked up too much of the background or it didn't pick up enough of the text.
The image you supplied is obviously a photograph taken in either April or September 2014 - the EXIF information is still attached - and the lighting is poor and that is your problem. It doesn't look as though the document was in a glass case but there is a terrible amount of shadow to the left.
I think you have only three effective solutions : re-scan the document, or re-shoot it this time using flash, or - if you must have clean text - transcribe the document. This will of course take away the effect of having the original handwritten document, and I must say the writing is very legible. I presume this document is in a library or museum somewhere - have you tried contacting them to see if they have or can make a clean copy for you ?