Because the links change every so often, Google search links are sometimes used instead.
For long time, Intel offers download of its x86 architecture manuals for free, called Software Developer Manuals. The most important are Instruction Set Reference manuals and Basic Architecture manual.
Google search: Intel Software Developer ManualsAMD offers download of its x86 architecture manuals for free as well as Intel, called Programmer's Manual. Application Programming manual serves as Intel's Basic Architecture manual. General-Purpose and System Instructions serves as Intel's Instruction Set Reference manuals.
Follow the link and then search for "Programmer's Manual" on the page.
Google search: AMD Architecture Programmer's ManualARM manuals are free too. Login (free) is required.
ARM Architecture Reference ManualsAvailable in DOCX format.
Google search: Microsoft Portable Executable and Common Object File Format SpecificationThe world's leading source for pure technical x86 processor information.
http://www.sandpile.org