I don't own the Tome myself, but the 3.5 and 3.0 stuff won't contain the Combat Manuevers Attack and Defense numbers, which are easily calculated off the cuff.
The monsters themselves will probably be slightly weaker than you would expect. Pathfinder characters tend to have slightly more skills and hp as well as big boosts to martial characters.
The nerf to the save or die spells won't really matter to you since its an across the board thing.
You should be fine, just be aware some skills were merged between editions. Like Spot is in Perception, Concentration is in Spellcraft, etc.
Edit: Most adventures should be simple to convert. The Role Playing won't change much, and you can easily grab the updated monsters from the online System Reference Document since most Pathfinder stuff is Open Game Content.
Pathfinder and 3.5e aren't identical. Compatible, yes, identical no.
For instance: creatures in Pathfinder get feats at every odd hit die, where in 3.5e they'd get feats every three hit dice. So you can use a 3.5e monster manual, but recognize that those monsters can be "missing" feats. Not hard to ignore or on-the-fly address.
For instance: skills are more consolidated in Pathfinder. You'll find monster statblocks from 3.5e reference Search, Spot, and Listen where in Pathfinder those are all just Perception. Again, you can use a 3.5e monster manual, but just on-the-fly adjust a few skills.
For instance: the combat maneuver system was completely rebuilt in Pathfinder. You'll find 3.5e monsters reference a Grapple stat. That doesn't exist in Pathfinder, where you have a CMB/CMD set of stats. Those are very easily calculated, so you can use a 3.5e monster manual and just on-the-fly adapt to the CMB/CMD system for grapples, bull rushes and so on.
For instance: classes in Pathfinder get something every level. There were a lot of "dead levels" in 3.5e where you'd get very little. Perhaps your BAB and saves went up, or you got a spell slot. Hardly exciting. Pathfinder added things like rage powers, rogue talents, sorcerer bloodline abilities, and cleric domain abilities to fill in those dead levels. You can use a 3.5e monster manual, but classed monsters will be missing those little filler features. It's very minor and easily ignored, but it's there.
That's a summary for you. The two systems are very, very compatible but there's still some minor tweaking involved. On the other and, using 3.5e material with 4e just doesn't work. Everything got rebuilt and abilities power-scale were remapped, so you'd have to basically rewrite everything from scratch.
Think of Pathfinder as 3.75e 'Cuz that's pretty much what it is. Good luck.
For instance: creatures in Pathfinder get feats at every odd hit die, where in 3.5e they'd get feats every three hit dice. So you can use a 3.5e monster manual, but recognize that those monsters can be "missing" feats. Not hard to ignore or on-the-fly address.
My general "on the fly" method is to just leave the monster completely as-is (computing CMB and CMD if I need it), and reducing CR by 1, sometimes 2. Overall, that worked OK when I was in the process of "cutting over" from 3.5 to Pathfinder.
As I have both, most monsters are ok to use the old, but some like undead and dragons are quite different. Undead has already been mentioned, dragons were nerfed in many ways on hit dice in pathfinder. I would recommend updating. It was the first pathfinder book I bought and in fact bought it before converting over to pathfinder.
You might also want to check for specific monsters on www.d20pfsrd.com
We updated all three Tomes of Horror to "Tome of Horrors Complete" and those updates are available there.
I have the Pathfinder Tome of Horrors Complete. In the preface it seems to imply it's just the same Tome of Horrors with those conversions done plus a couple extras for wholesome goodness. Following that you should be fine.