Most units of measurement are abbreviated: a 5cm gap; she drove at 70mph.
But not those units expressing distance: He fell 20 metres; we walked 150 miles.
Use abbreviations for sums of money: £1bn. But: one million people.
Some common titles are abbreviated (see Names and titles).
Abbreviations do not take a full point.
Use all capitals if an abbreviation is pronounced as the individual letters: BBC, VAT etc; if it is an acronym (pronounced as a word) spell out with initial capital, eg Nasa, Nato, Ucas unless it can be considered to have entered the language as an everyday word, such as awol, laser and, more recently, asbo, pin number and sim card.
Note that pdf and plc are lowercase.
Etc and eg without full points.