Proficiency Without Level

This campaign is using the Proficiency Without Level (PWL) variant rule covered in the Game Mastery Guide on page 198. This is the Pathfinder 2E version of "bounded accuracy" described in the D&D 5th edition version.
The primary reason for using the PWL rule variant is to allow a broader range of challenges at any given level. Low level characters can face higher level challenges, and lower level creatures can still challenge higher level characters.
This is a very significant change to the fundamental mechanics. Making PWL scale well from 1st to 20th level depends on the GM applying Difficulty Class (DC) adjustments. However, this does mean the DCs tend to be more subjective.

Reference

The fundamental math I'm using is based on the rules variant, but tweaked as outlined from the community using this Google Sheets doc: PWL Resources.

Proficiency Bonus

Skill DCs

${}^{\ast }$ Default level DCs.

Adjusting Difficulty

DCs by Level

${}^{\ast }$ Default level DCs. Useful if you need to convert on the fly a feat DC or something else.

Adjusting Encounters

Encounter Budget

Non-Standard Party Sizes

The encounter budget is based on a group of four characters. For every character above or below four you add or subtract the amount listed above for character adjustment (there is a minor change in this table from the core rules for low threats).
In the core rules an encounter is created using the XP budget adjusted by the number of characters above or below four, but the actual XP award given to the players are the same as the XP budget for a group of four characters (i.e. a Moderate encounter with an XP budget of 160 for eight characters still results in 80 XP for each character).
This can be difficult to work out for encounters with a large number of enemies and allies. To simplify the calculation after an encounter the following formula will be used:
XP Award = (Sum of XP for all monsters) x 4 / Number of characters

Treat Wounds

${}^{\ast }$ Rolling against a higher DC is optional.

Learn a Spell

Demolishing Barriers

Rule Adjustments

Aid

The base DC is 18.

Assurance

Even in the worse circumstances, you can perform basic tasks. Choose a skill you're trained in. You can forego rolling a skill check for that skill to instead receive a result of 10 + your proficiency bonus + Ability Modifier.

Animal Form (spell 2)

When calculating the stat for your animal follow these rules:
AC: Don't add your level
Attack Modifier and Athletics: Remove from the listed bonus the spell level times 2.
Example: Heightened (4th) (value to remove is 4 times 2 = 8) Your battle form is Large and your attacks have 10-foot reach. You must have enough spece to expand into or the spell is lost. You instead gain 15 temporary HP, AC = 18, attack modifier +8, damage bonus +9, and Athletics +8.

Long Jump

The DC is 18.

High Jump

The DC is 21.

Untrained Improvisation (General Feat)

You’ve learned how to handle situations when you are out of your depth. Your proficiency penalty to untrained skill checks is equal to -0 instead of -2. This doesn’t allow you to use the skill’s trained actions.