Firstly you will need a small picture of the pdf logo uploaded to your server. Once this is done the you add the follwing code
Code:
<img src="url to pdf logo here"><a href="url">Text to be displayed here</a>
So for example if you were to do the right click, view html option on a page with the above code the page would look something like
If you wanted to change the size to the image you would add code as follows
If you want there to be alternate text in place of the picture when the browser can't load the picture the following code would be entered instead
Code:
<img src="url to pdf logo here"alt="alternate text goes here"><a href="url">Text to be displayed here</a>
In answer to your other question. To change the HTML of a page, presuming it's your own you would find the file on your server and either edit via a text editor like notepad or other editors like Bluevoda. If you are trying to copy code from websites that are not your own via the right click, view source option, be very wary as there are often copyright issues with doing this.
A good source of information on HTML programming can be found at W3Schools.com
Hope this helps