My late grandfather passed on the family tree to me, included in everything is a WW1 war journal, leather bound notebook and original pencil, both belonged to my great-great uncle who was killed on the western front, and family portraits from before he went to war.
The journal is in amazing condition, he kept a detailed log of his movements up until his death.
The notebook is slightly worn but still great condition for 98 years old.
I don't know how I should be storing these to maintain the condition. Right now they're in a plastic folder...
This might be a good question for /r/Libraries/
The first thing before you go to that last commenters site, is to get it out of the PLASTIC binder. The PLASTIC has compounds that eat up the pages. Good luck, that is a great gift. Oh, a comic book store has clear envelopes that are not harmful to paper for storage.
Acid free archival box wrapped in acid free preservation paper. Only handle it with cotton gloves . Acid free paper between the pages if they are sticking or showing signs of rub off. And get it out of the plastic asap.
Keep it in a low humidity areas with no direct sunlight.
As someone else mentioned scanning all pages will help preserve there contents in a digital sense.
Good luck!
Scanning it and putting it online is another good way of preserving it.