A PDF is a format rather than a type of resource. You should reference this according to the type of source, but if this is not possible, you can reference as a website.
The nature of web resources means that author names are often not available and dates can be very vague. Therefore you will need to decide who is responsible for producing the web page and use them as the author, often this will be an organization rather than a personal name. You should be able to find this information by following “about us” or “contact us” links. If there is no author, or organization you can use the title of the web page, if there is no title use a truncated web address (you should consider whether this resource is suitable for academic work).
Author(s), Title of document, Organization/publisher, year. [Online] Available from: web address [Accessed date].
e.g. Institution of Mechanical Engineers, Energy and buildings: energy policy statement 09/05, Institution of Mechanical Engineers, 2013. [Online] Available from: http://www.imeche.org/knowledge/policy/energy/policy/energy-and-buildings [Accessed 13/05/13].
Wiki name, Title of article, [Online] year. Available from: web address [Accessed date].
e.g. Rapid prototyping, Reprap, [Online] 2009. Available from: http://rapidprototyping.wikidot.com/reprep [Accessed 24/05/13].
Author, “Title of the posting” (if applicable), Title of the site. [Online] day/month/year of posting. Available from: web address [Accessed date].
e.g. W. Osama, “DHCP client testing tool”, Networkers online. [Online] 26th April 2013. Available from: http://www.networkers-online.com/blog/ [Accessed 17/05/13].
Originator, Description or title of image [Online image], year. Available from: web address [Accessed date].
e.g. Electrical Engineering Portal, Electrical engineering books and guides [Online image], 2013. Available from: http://electrical-engineering-portal.com/download-center/books-and-guides [Accessed 01/07/2013].
Lecturer(s), “Lecture title”, from module code (capitalised) title of module. Teaching organisation, location and date of presentation. Available from Blackboard [Accessed date].
e.g. S. Linfoot, J. Gow, and P. Prickett, “Introductory Lecture”, from ENGD1008 Principles of design and manufacture. De Montfort University, Queens Building on 11th October 2012. Available from Blackboard [Accessed 03/07/13].
Individual authors are rarely acknowledged. If you cannot find a named author of an electronic source then use the organization or title in place of the author.
Author or Originator, Title, [Software] version/series etc. Place of publication: Publisher, year.
e.g. SPSS, SPSS for Windows, [Software] version 12.0.1. Chicago: SPSS, 2004.
Note: Journal articles taken from the Internet or a database should be cited as print using the rules for print journals. However if the article is only available online (for example there are no page numbers) reference as below.
AUTHOR(S), "Title of article", Title of journal, [Online] Volume no, Issue no, month and year of publication. Available from: DOI or web address [Accessed date].