I am running MacOS 10.7.5 and FMP12v2.
I normally don't try to change your subject question but I am unclear on why you want to open the PDF file.
It appears you want to use the PDF as a Historic file of Membership information.
If this is True...
Then simply save a new PDF files with the Updated Membership info and use the Same file name in the same Directory and Mac will replace old PDF file.
PDF files can be viewed [and rough printed] from container fields in FMP, but the information inside a PDF cannot be edited in FMP.
A PDF file created by FMP is not changeable after the conversion to PDF by FMP. It can be replaced as i said above.
Mac does have a PDF reader licensed by Adobe called Preview. You can open Preview and use the PDF file there, or Adobe reader.
Remember that FMP is named File Maker for a reason. It stores the Membership information in Records and is "simulating" a file folder/file cabinet system [plus much more since its conception 12 changes back in years]. Then you can create a near paperless file and data manipulations program using FMP.
I use PDF's in FMP to...
1) Store external generated PDF files for historical and reference purpose.
2) Generate custom internal office forms in PDF format to email, print, save for [unchangeable] legal reasons.
3) Store embedded PDF's in FMP for visual quick reference in FMP 11 and earlier versions or as an external File reference in FMP12.
I only "open" a PDF using Preview or Adobe to print a crisp printed copy or group a series of PDF to bundle them from several PDF's referenced in FMP.
To program a button to "open" an external PDF file, I would need to know how the PDF file is referenced or stored in you FMP database.
PS: what is your MacOS and FMP version?
Where did this PDF file come from? If this is data originally stored in your database, why save it as a PDF?