If you want the PDF to be displayed in the same mashup beside the button, follow the article to use a WebFrame, as mentioned in the above post.
If you want to open PDF in a new window or download it, then you can use a Link widget. Set its visible property to false. Bind the Clicked event of button to the Navigate property of the Link widget and specify the URL to the PDF directly in the LinkURL property or bind a value to it using a service.
If the PDF is available in a TWX FileRepository, then
1. Use below URL format if you want to open PDF in a browser window
https://[TWX_HOST]:[TWX_PORT]/Thingworx/FileRepositoryDownloader?download-repository=[REPOSITORY_THING_NAME]&directRender=true&download-path=[PATH_TO_FILE].pdf
2. Use below URL format if you want to download PDF upon clicking button
https://[TWX_HOST]:[TWX_PORT]/Thingworx/FileRepositories/[REPOSITORY_THING_NAME]/[PATH_TO_FILE].pdf