Finding Optimal PDF Paper Size for Kobo Clara HD Ereader

tl;dr: Optimal page layout for Kobo Clara HD ereader: Width: 9cm, Height: 11.1cm, margins: 0.5cm.

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I have recently bought a Kobo Clara HD ereader. I used to own a Glo HD ereader by the same company, until I accidentally pushed it off the table causing its display to crack.

Though I prefer EPUB format for reading on an ereader, there are many cases when a PDF format might be more convenient. However, regular PDF files are difficult to read on the ereader for their large page sizes. The PDF viewer of the device scales a page to fit its display causing tiny texts. I can zoom in a section to read, however I need to swipe and drag around to view other areas which is really not convenient due to slow response time of e-ink display. Worse is when I accidentlly tap the display, causing it  to immediately turn the next page. I need to go back to previous page and go through all the zooming and dragging before I can resume reading. Therefore I always wanted to generate a PDF optimized for my ereader so that I will not have to go through all those hassles.

The PDF viewer of the ereader reserves a small bar at the bottom to display page number and renders the pages of a PDF in the remaining areas. Each page is scaled to fit the render area both horizontally and vertically. Therefore, in order to avoid scaling, the page size of the PDF should match the render area perfectly.

I managed to find the optimal page size after several trial and error. For this purpose, I used the Microsoft Word software. The general workflow is, I export a document to PDF, load it on to the device, open it in the device and watch the scaling of a page. The reason I used MS Word is that I can quickly and easily change the paper size and save the document as PDF.

I started with A6 paper size and 5mm margin as advised in a stackexchange post. I saved the document as PDF, loaded the pdf on to the ereader and opened it using built in PDF viewer of the device. At first glance, the margin on the sides looked wider than the top. I made the conclusion that the page height must be greater than the render area causing it to scale down and the appearance of wider margin on the sides. However, I could not tell the actual scaling since there were no page border.

I came back to the Word document and decreased its page height. I also added a solid page border (from the Design ribbon) at 1pt from the edges. I left 1pt gap from the edge because I thought the border would be difficult to see alongside black bezels of the ereader. I saved the document as PDF, loaded it on to the ereader and opened it again to verify. I found that the top border is at the top end of the display, there were thin space outside the left and right border and relatively large space below the bottom border.

I realised that, the bottom border is irrelevent and I only need to compare the position of top and side borders. For a perfect fit, the top border and the side borders need to be equally apart from their respective bezels. Of course, this does not mean that pages are not being scaled, only that the aspect ratio of the page and the render area match and we can utilise maximum area of the display.

After several trial and error, I found the optimal page size which fits perfectly to the display area. In between the trials, I also measured the display size physically with a scale, therefore I'm pretty sure that it is a perfect fit and no scaling is being done. The optimal page size for KOBO Clara HD I found is Width: 9cm, Height: 11.1cm

For my future reference, and for anyone else with a different ereader, here is the summary of the steps I followed:

1. Prepare the PDF (or download from here)
• Open a new Word document
• Change page size to A6 (Page Layout ribbon)
• Change all margin to 5mm (Page Layout ribbon)
• Add a solid page border at 1pt from the edge (Design ribbon)
2. Save the document as PDF
3. Load the PDF file on to ereader
4. Open the PDF file on ereader
5. Check top, left and right borders. They all should be at the edge of the display for a perfect fit.
6. Adjust the page size accordingly. Repeat the steps until a satisfactory output page size is found.
7. TIP: Physically measure the width and height of the render area. Width is the same as the width of the ereader display. For measuring the height, make sure the top border is at the edge of the display by selecting a tall page size, then measure the distance between top and bottom border.