None, one letter
Split by number of pages
Split by phrase presence

Click the "Split Pages Every" field and enter the number of pages in each document. 

In this example, each letter contains a single page, so we will set this to "1"

Click this dropdown to see each individual letter

Letter 1
Letter 2
Letter 3
Letter 4
Letter 5
Letter 6
Letter 7
Letter 8
Letter 9
Letter 10
Letter 11
Letter 12
Letter 13
Letter 14
Letter 15
Letter 16
Letter 17
Letter 18
Letter 19
Letter 20
Letter 21
Letter 22
Letter 23
Letter 24
Letter 25
Letter 26
Letter 27
Letter 28
Letter 29
Letter 30
Letter 31
Letter 32
Letter 33
Letter 34
Letter 35
Letter 36
Letter 37
Letter 38
Letter 39
Letter 40
Letter 41
Letter 42
Letter 43
Letter 44
Letter 45
Letter 46
Letter 47
Letter 48
Letter 49
Letter 50
Letter 51
Letter 52
Letter 53
Letter 54
Letter 55
Letter 56
Letter 57
Letter 58
Letter 59
Letter 60
Letter 61
Letter 62
Letter 63
Letter 64
Letter 65
Letter 66
Letter 67
Letter 68
Letter 69
Letter 70
Letter 71
Letter 72
Letter 73
Letter 74
Letter 75
Letter 76
Letter 77
Letter 78
Letter 79
Letter 80
Letter 81
Letter 82
Letter 83
Letter 84
Letter 85
Letter 86
Letter 87
Letter 88
Letter 89
Letter 90
Letter 91
Letter 92
Letter 93
Letter 94
Letter 95
Letter 96
Letter 97
Letter 98
Letter 99
Letter 100

That's it. We have just split 1 pdf into 100 letters, each containing a single paged letter. 

How to split a document by a fixed number of pages

Daniel made this Scribe in 24 seconds.