Free and paid woodworking plans can both be excellent. The useful difference is whether the plan gives you enough information for the project you want to build.
What a good plan should answer
Before choosing by price, check the dimensions, material list, cut information, tool requirements, drawings, joinery details, and build sequence. A paid PDF can still be incomplete. A free plan can be carefully documented.
When free plans make sense
Free plans are useful when you are exploring a project type, want a straightforward build, or prefer to evaluate one project at a time. They also let you see how a source explains measurements and assembly before you commit money to a larger collection.
When paying can make sense
A paid plan can be worthwhile when it saves enough searching or design work to justify the cost. A larger paid library may appeal when you build often and value having many project directions in one place. The size of a library does not prove that every plan will suit your shop, so inspect representative material and current terms whenever possible.
Use the same checklist either way
Check the finished size, material thickness, joinery, required tools, hardware, drawings, assembly order, and any assumptions the author makes. Read the whole plan before buying lumber.