If you have a valid serial number, you can obtain SolidWorks 2018 SP5 through these standard methods:
SolidWorks does not offer a permanently free "Premium" version for general use, but there are legitimate ways to access the software at no or low cost:
: SolidWorks 2018 SP5 is notable for providing "Previous Release Interoperability," allowing it to open parts and assemblies created in SolidWorks 2019 with limited functionality. Legal "Free" Options