A gedcom file is just a text file that follows certain specifications. Try opening the gedcom from the File menu of in a text editor or word processor, (or right-click the file and select Open With ... and select a text editor such as Notepad).
It should start off like this:
0HEAD 1 SOUR PAF 2 NAME Personal Ancestral File 2 VERS 126.96.36.199 2 CORP The Church of Jesus Christ of Latter-day Saints 3 ADDR 50 East North Temple Street 4 CONT Salt Lake City, UT 84150 4 CONT USA 1 DEST PAF 1 DATE 27 Feb 2009 2 TIME 08:57:16 1 FILE [name of your file].ged 1 GEDC 2 VERS 5.5 2 FORM LINEAGE-LINKED
If your gedcom does not start like this, it is not a gedcom file.
The basic procedure in PAF is File menu > Export > Partial > Select > (select the deceased individuals to include) > OK > (remove check marks from any data you do not want included) > Export > (select a name and location for the file, removing the "*" from the name) > Export > OK.
If you want to include everyone, you can skip the selection process, but you don't want to upload living people's data to Ancestry.