Gmail offers search feature similar to Google search. it is very easy to use.
If you are looking for an attachment and if you know the name you can do the search by the following method.
This search will bring all the emails with the attachment timesheet
You can also do this search for file types, If you know that you have a PDF and you do not know the name, you can type Filename:PDF
This search will bring all the emails with the PDF attachment
There should not be any space in between File and Name
You can also do this search by going to the search options screen
You will see the screen as shown below where you can do the detailed search without entering the advanced operators
If you have any questions or suggestions, please post a comment.