If you are not sure if the website you want to visit is safe, you can check it out here. Enter the address (url) of the website and see the invisible elements, such as a header or meta data.
site address:www.headfirstlabs.com/books/hfsql/HFSQL_subquery_excerpt.pd...
URL does not point to a web page (html/xhtml) or read error or page not found. (6001)
Url ContentAdobe Portable Document Format
MIME Type / Internet Media Typeapplication/pdf
MIME Type application
File Extension.pdf
More Details (link to:)Adobe PDF
Powered by: Very Tiny URL Shortener at https://vturl.net VeryTinyURL

Header

HTTP/1.1 200 OK
Date Sun, 21 Jan 2018 18:36:00 GMT
Server Apache
X-Content-Type-Options nosniff
Last-Modified Fri, 31 Aug 2007 15:10:46 GMT
ETag 20a0a5f-6ffb43-439003a324980
Accept-Ranges bytes
Content-Length 7338819
Content-Type application/pdf

Load Info

page size7338819
load time (s)3.071543
redirect count0
speed download0
server IP65.212.180.43


* all occurrences of the string "http://" have been changed to "htt???/"