Yes, 7-Zip is free software. You can use it on any computer. You don't need to register or pay for 7-Zip.
You must run 7-Zip File Manager in administrator mode. Right-click the icon of 7-Zip File Manager, and then click Run as administrator. Then you can change file associations and some other options.
New versions of 7-Zip (starting from version 15.06) use another file sorting order by default for solid 7z archives.
Old version of 7-Zip (before version 15.06) used file sorting "by type" ("by extension").
New version of 7-Zip supports two sorting orders:
You can get big difference in compression ratio for different sorting methods, if dictionary size is smaller than total size of files. If there are similar files in different folders, the sorting "by type" can provide better compression ratio in some cases.
Note that sorting "by type" has some drawbacks. For example, NTFS volumes use sorting order "by name", so if an archive uses another sorting, then the speed of some operations for files with unusual order can fall on HDD devices (HDDs have low speed for "seek" operations).
You can increase compression ratio with the following methods:
If you think that unusual file order is not problem for you, and if better compression ratio with small dictionary is more important for you, use 'qs' mode.
In 99% of these cases it means that the archive contains incorrect headers. Other ZIP programs can open some archives with incorrect headers, since these programs just ignore errors.
If you have such archive, please don't call the 7-Zip developers about it. Instead try to find the program that was used to create the archive and inform the developers of that program that their software is not ZIP-compatible.
There are also some ZIP archives that were encoded with methods unsupported by 7-Zip, for example, WAVPack (WinZip).
7-Zip doesn't know folder path of drop target. Only Windows Explorer knows exact drop target. And Windows Explorer needs files (drag source) as decompressed files on disk. So 7-Zip extracts files from archive to temp folder and then 7-Zip notifies Windows Explorer about paths of these temp files. Then Windows Explorer copies these files to drop target folder.
To avoid temp file usage, you can use Extract command of 7-Zip or drag-and-drop from 7-Zip to 7-Zip.
You're probably using a *.* wildcard. 7-Zip doesn't use the operating system's wildcard mask parser, and consequently treats *.* as any file that has an extension. To process all files you must use the * wildcard instead or omit the wildcard altogether.
In most cases you don't need -r switch. 7-Zip can compress subfolders even without -r switch.
7z.exe a c:\a.7z "C:\Program Files"
compresses "C:\Program Files" completely, including all subfolders.
7z.exe a -r c:\a.7z "C:\Program Files"
searches and compresses "Program Files" in all subfolders of C:\ (for example, in "C:\WINDOWS").
If you need to compress only files with some extension, you can use -r switch:
7z a -r c:\a.zip c:\dir\*.txt
compresses all *.txt files from folder c:\dir\ and all it's subfolders.
32-bit Windows allocates only 2 GB of virtual space per one application. Also that block of 2 GB can be fragmented (for example, by some DLL file), so 7-Zip can't allocate one big contiguous block of virtual space. There are no such limitations in 64-bit Windows. So you can use any dictionary in Windows x64, if you have required amount of physical RAM.
For exe installer: Use the "/S" parameter to do a silent installation and the "/D=dir" parameter to specify the "output directory". These options are case-sensitive.
For msi installer: Use the /q INSTALLDIR="C:\Program Files\7-Zip" parameters.
There are some possible cases when archive is corrupted:
It's possible to recover some data. Read about recovering procedure: