I have just downloaded OrionSDK-1.9.zip and extracted the msi, but there is no trace of pdf file for the documentation.
After installing the msi , I found it in C:\Program Files (x86)\SolarWinds\Orion SDK\Documentation as Orion SDK.pdf.
Just in case anybody is wondering about that.
The current release of the Orion SDK is not 1.9; it's 2.1 - Releases · solarwinds/OrionSDK · GitHub
The information from that PDF has been moved to the GitHub wiki and expanded. I recommend you rely on that instead of the pdf. Home · solarwinds/OrionSDK Wiki · GitHub