downloaded Adobe Reader 11.0.16 but cannot open pdf - message "because it is from an unidentified developer"

Have imac with Yosemite 10.10.4 and recently updated Adobe Reader to 11.0.16. I need to open an application form with A.R. to enter information but after changing my default setting from Preview for this file only I get the message "pdf cannot be opened because it is from an unidentified developer". I know nothing about changing security settings, which is also mentioned, and don't know if the offender is A.R. or the pdf file. The file to open is from the UK Heritage Lotttery Fund, a very large reputable organisation, who advised me to use A.R. when I told them that when I had used Preview the information boxes would not expand as they should when text is entered, neither would it. I don't think this will be an uncommon problem and would be very grateful for advice as it is rather urgent.

Thanks, Arnold