The newer version files of Solidworks (2019 & 2020) can't be viewed on 2018 version. What can be done without having to download the new version?
short answer is, you cant.
But, if I remember correctly, you can view 2019 sp1 on 2018sp5, you can view 2020sp1 on 2019sp5. The last sp that was released (sp5) can see sp1 of the next years release
And they just looks like an import.
If you have 2018 SP 5 installed you can view 2019 stuff, but you're limited to viewing.
Depending on what you're trying to do, you may not have any other option except to upgrade.
To view the model you can export the 2019/2020 3D files to a dumb solid (STEP or something) - if you have access to 2019/2020 - and re-open in 2018. That will at least let you view and edit dumb-solids of all the geometry; but if you need to edit or access original SW files created/saved in 2019/2020 there isn't going to be an alternative to updating versions.
2019v is opened in 2018v this is what you can see.
You can install current version of free eDrawings to get some info out of newer files.
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