How to reference a PDF document found on a website I want to use this website in my report but I do not see the produced year of the following pdf https://www.astm.org/BOOKSTORE/DS68/pg41.pdf

How can I do the reference of that pdf ?

Is this correct : ASTM international, "Ultrasonic Testing," ASTM international,. [Online]. Available: https://www.astm.org/BOOKSTORE/DS68/pg41.pdf. [Accessed September 2016].
Since this is a book, you will probably be asked to give the book citation instead of the link. In this case you reference the book as indicated by the journal and refer to the page inline.

e.g. We used the water attenuation as a function of frequency from Book_Author, year (Table 2.7, p. 41). 

In the references use the journal's style, or for your report, use the style you reference papers. Then use the publisher of the book instead of the journal name (e.g. Cambridge University press)

This is usually done because a link might be broken after some years and then the reader won't be able to find the reference. In science, links are usually used when referring to some online extra material (e.g. videos, tables, codes, datasets) that the authors uploads to their own webpages, besides the online material provided to the journal. This is usually done because they want the data to be publicly available and the online material on the journal webpage might not be.

Even if your question is about a report and not a scientific journal, the best way to approach this is to learn to proper way to do it and stick to it. This will make your texts more professional and when it comes to papers you will be less likely to have cited, captioned etc something in the wrong way. The latter will help you have shorter referee report with less corrections.