Start by identifying the actual limit
Write down the original file size, triangle count if available, intended print dimensions and the upload limit. A file rejected by a website needs a different response from an object that is physically too large for the bed. Scaling down coordinates does not remove triangles from a binary STL.
Keep a master copy and create a working revision. Do not overwrite the only detailed model to satisfy a temporary upload restriction.
Try a compact encoding before simplifying
STL can be written as text or binary. The Library of Congress documents the more compact binary representation. Re-exporting a text STL as binary can reduce storage without deliberately removing faces; this will not help much if the source is already binary.
A ZIP archive may help transfer a file, but only if the destination accepts archives. Unpacking it restores the same mesh complexity. Renaming the extension does not convert the encoding or simplify anything.
Reduce geometry gradually, not to a magic percentage
Blender’s Decimate modifier can reduce mesh complexity. Its ratio controls how much geometry is retained; the setting is not a quality guarantee. Work on a duplicate and compare each candidate from the same viewing angles.
Mark three details that matter before editing: for example an engraved letter, a rounded rim and a narrow connector. Broad nearly flat surfaces may tolerate simplification better than those features. Stop when an important edge changes, even if the file is still above your target. The cover is a conceptual illustration, not a measured benchmark.
Validate the exported revision
Export the intended evaluated geometry, reopen the STL and compare its file size and dimensions with your notes. If the modifier appears to have made no difference, check the export settings and selected object before applying stronger reduction.
Slice with the intended printer profile. Examine the small features, perimeter paths and connections at the final scale. A smaller file is not proof of a sound mesh or successful print. If the only acceptable reduction destroys a required detail, change the transfer method or workflow instead of concealing the loss.