Running my first Pathfinder game and was wondering if I am able to use the monsters in the D&D 5e Monster Manual in encounters in Pathfinder. I just feel the D&D monster book's layout is more readable and user friendly. Plus, I was given this book as a present even though I play Pathfinder. Everything looks similar, I just didn't know if it is possible.
Not directly, no.
Pathfinder and 5e work on very different rulesets, including different methods of calculating attack, defense, and damage values, and different expectations of the PCs capabilities. Saves work differently, feats work differently, many monster abilities and effects work differently, etc.
While the stats look similar on the superficial level, you'll begin to run into problems when you expect a monster to be a threat to your characters, only to realize that its AC and attack bonuses are so low that the PCs can't miss it, and it can't hit them.
For example (picking a creature at random): in Pathfinder, an Ancient Green Dragon has an AC of 36 and an attack bonus of +31. In D&D 5E, an Ancient Green Dragon has an AC of 21 and an attack bonus of +15. You'll find similar huge discrepancies all throughout, because the underlying math of the two games is very different.
To make this work, you would need to find a way of converting 5E math to Pathfinder math. I've been able to find methods of converting Pathfinder to 5E on Google, but not the other way around.