Consider taking a look at the Black Streams Solo Heroes rules (free) and/or the Scarlet Heroes ruleset (not free), by Kevin Crawford. They adapt D&D to work with a single character, and are designed to handle all the necessary adjustments at a system level, so you don't have to worry about manually adapting each module you encounter. Black Streams is an addon designed to be used with your d&d system of choice; Scarlet Heroes uses its ideas and builds a self-contained ruleset out of it. There is, however, one major issue: they're written for the really old school editions of the game, so some numbers will need adjusting.
To briefly summarise what the rules do:
First, damage is dramatically reduced. A roll of 1 does no damage, a roll of 2-5 does 1 damage, 6-9 does 2 and more than that does 4. The PC takes this reduced damage to his hp as usual, but enemies effectively have a hp equal to their number of hit dice. So an orc with 2 hit dice who gets hit for 6 damage (pre-reduction) is immediately killed. There are also some additional rules regarding multiple damage dice, and what happens when you take a flat amount of damage rather than rolling. Note that both damage and hp-per-hit-dice are a good deal lower in the old school rules, so these numbers will need adjusting: perhaps halving the PC's hit points (or giving enemies 2hp per HD instead of 1) and then adjusting how much damage gets reduced by according to taste. This also ignores the effects of a high constitution on NPCs and monsters; YMMV on whether you want to come up with a way to adjust for that in some situations.
Healing magic works the same way in reverse; and after a fight you can also take a few moments to catch your breath and restore a few of the HP lost in that fight. This helps if you don't have healing magic of your own, though again the number of HP restored would definitely need to change for Pathfinder. Perhaps 1HP per level might work.
Secondly, if the PC manages to kill an enemy, excess damage can carry over and hit more enemies. Sort of like the 3.5 version of the great cleave feat, except that you don't reroll the attack roll, you just deal the damage left over after your last kill. To avoid exploits, the new target can't have a better AC than your original one.
Thirdly, you're given something called a "fray dice": an automatic dice of damage you can inflict every round, even if you miss with your main attack or are doing something else. This means you're still killing the enemies even if you have a string of bad rolls with all your other attacks, which can happen far more often when you only have one or two a round which hit really hard rather than an entire party's worth that do relatively little damage. There are a few restrictions on what you can hit with it, though: it's for dealing with mooks, not killing bosses. In the original rules, the dice size depends solely on your class; in Pathfinder, though, it should probably scale with your level in some way.
Fourthly, the system provides a mechanic designed to help when they don't have the magic/skill points/etc that a party would normally have access to, or when they're hit by a nasty single-target disable that would normally be dealt with by other party members: defying death. When the PC is in a situation they couldn't otherwise solve (failed a save against a save-or-lose spell, don't have the ability to fly up and pick up a plot coupon, etc) they can invoke this ability to get out of the situation. Perhaps they resist the spell after all, or they spring up to the ceiling in a single mighty bound, snatching the plot coupon as they do so. In return for this, they take a level-based amount of damage, which increases the more times you use the ability in a given day.
Scarlet heroes also switches most d20 rolls (with the notable exception of attack rolls) to 2d8, with target numbers adjusted appropriately. This gives a bell-curve shaped distribution, which helps make things less "swingy": since you'll often find that success or failure depends on a single dice roll, rather than several from different PCs, the normal rules tend to produce rather extreme results more often than you might like.