With PubHTML5 you can define the page number caption in the following way:
Step1. Open PubHTML5 and import your PDF or image file.
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Step2. Go to the "Custom Settings" tab and choose the "Simple Mode" option.
Step3. Find the "Page Numeration" option by searching it in the "Search" field.
Step4. Set the page number caption as you wish.
Step5. Click the "Apply Change" button on the top toolbar to save the change.
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Note: The format is: actual page number:caption. For example, if you set 1:I; 2:II; 3:III; 4:IV; 5:V; 6:First Unit; 7:;8-100:1+, it will show I for the first page, II the second page, III for the third page, IV for the forth page, V for the fifth page, First Unit for the sixth page, empty/nothing shown for the seventh page, then show 1, 2, 3...93 for page 8 to 100 (1+ means show caption from number 1, and increase 1 corresponding to the later pages). Another example, you have 30-page book and you want to show page numbers from 3001 to 3030, then you can set caption as 1-30:3001+.
The caption value also can be set with Macro: Start Page Index-End Page Index:any string{N/R/r + Start Number}any other string (N means Arabic number, R means uppercase Roman number, r means lowercase Roman number, + means increase progressively). For example, if you want to show page numbers for a 1000-page book as P6, P7, P8...P1005, you may use this: 1-1000:P{N+6}; if you want to show page number for a 10-page book like Page I#, Page II#, Page III#, Page IV#... Page X#, you can simply use 1-10:Page {R+}#.