There are a few challenges one needs to meet to solve this problem.
First, creating a book in the 14th century was an expensive undertaking. All of the pieces -- the writing material, the ink, the process of writing the document -- had to be done by hand by skilled workers. I don't remember the costs offhand -- although I believe they have been calculated -- but it would require a lot of money. (Can your time traveler bring with him several pounds of gold to pay for these expenses?)
Second, there are two different strategies in preserving an object like this: one is passive, the other active. Each has its strengths & weaknesses. The passive requires finding a spot that would be secure for 700 years. Its strength is security thru obscurity: by being hid, no one would know about it before you wanted it to be revealed. Its weakness is that if something happens to the object -- an accident, theft, damage due to age -- there is no way to right the wrong.
Your time traveler could secure this object in a durable container, & place it in a remote location. The choices would either be somewhere dry (e.g. the Sahara or Arabian Deserts), or cold & wet (e.g. in the permafrost of Siberia or North America). Such places would have a stable environment, & the materials of the manuscript would not deteriorate. (Keeping paper or parchment soaked in an anaerobic environment & frozen will preserve it. That's how written documents found near Hadrian's Wall dating from AD 100 survived.)
The problem with either option is for your time traveler to get to those remote locations in the 14th century. Once accomplished, then there is the challenge of marking the location the manuscript is secreted so it can be found when he returns to the present.
The active would be to entrust this to a family that guard it for that time. The strength here is that there would always be someone to respond in case something happened to the object: repair it if there was an accident or damage from age, protect it from theft, etc. The weakness is that, well, families tend to die out. A certain percentage of couples have no children; another will have only female children, & women had fewer rights for that time. And all it takes is for one irresponsible jerk to come into his inheritance for the object to be destroyed.
Your time traveler could identify a family that is known to have survived down to modern times, which would prevent some of this. But then you encounter the problem others have pointed out above -- since your time traveler has effected the time line simply by going back in time, you have no guarantee this family will actually survive down to modern times.
These solutions have been provided as a thought experiment. To actually implement any of them would require more thought & some careful research. (Or settling on an oddball solution, such as sealing the manuscript in a metal box welded shut & depositing it in the Cairo Geniza. Or have your time traveler create the Voynich Manuscript, thus knowing the key that decodes this mysterious document.)