When you've made a selection of addresses from the Customer Finder service then you download it in .CSV format.
You can use many different applications to open a .CSV including text editors such as Notepad, Sublime or TextEdit; as well as Microsoft Excel. (Bear in mind, though, that you have to open the file as .CSV in order to prevent Excel from dropping leading '0' characters on some numeric fields.)
The file format gives you standard Royal Mail PAF file columns, with column headings in the first row.
Here is an example of the first few rows of a sample download:
BuildingNumber,BuildingName,SubBuildingName,Thoroughfare,PostTown,Postcode,POBox,DeliveryPointSuffix,DependentLocality,DependentThoroughfare,DoubleDependentLocality
38,Hatfield Delivery Office,,Town Centre,HATFIELD,AL100AA,,1A,,,
1,,,Longmead,HATFIELD,AL100AB,,1A,,,
11,,,Longmead,HATFIELD,AL100AB,,1B,,,
13,,,Longmead,HATFIELD,AL100AB,,1D,,,
15,,,Longmead,HATFIELD,AL100AB,,1E,,,
17,,,Longmead,HATFIELD,AL100AB,,1F,,,