You can specify the document numbering sequence and number pattern to fit your need.
Click menu "Customize Document" > "Document Number". You can choose from different document types.
The "Document Number" dialog is shown.
Specify your numbering pattern.
0 - treat as the leading zero.
# - treat as the current document number
{YYYY} - 4-digits current year
{YY} - 2-digits current year
{M} - current month
Example 1
Numbering Pattern : INV#
Current Number : 2 => Output : INV2
Current Number : 23 => Output : INV23
Current Number : 345678 => Output : INV345678
Example 2
Numbering Pattern : INV0000#
Current Number : 2 => Output : INV00002
Current Number : 23 => Output : INV00023
Current Number : 345678 => Output : INV345678
Example 3
Numbering Pattern : INV{YYYY}0000#
Current Number : 2 => Output : INV201800002
Current Number : 23 => Output : INV201800023
Current Number : 345678 => Output : INV2018345678