SumatraPDF executable is compressed with executable compressor. It makes the file smaller and faster to download. On the downside, it increases probability of false positive from virus scanners.
Here’s a comparison using upx 3.07w and mpress 2.17 when compressing release version of r2466 (pre-release for 1.3):
4598272 uncompressed 4527104 stripreloc /b /c 2205184 upx —best —compress-icons=0h 1756160 upx —ultra-brute —compress-icons=0 1725440 mpress -s (-s : find the best compression) 1735168 mpress -s -r (-r : dont compress resources) 1767424 mpress
The last result is for StripReloc, which is not a compressor. It only removes unneeded .reloc section in exes. Mpress must already be doing that internally because compressing raw version and stripreloc’ed resulted in the same size.
There is a serious downside to executable compressors: many anti-virus program falsely report compressed executables as a virus or malware.