About PDF Bookmarks, Zoom factors, and page sizes

Bookmarks and zoom factors

The original idea of a PDF bookmark is that the person viewing a PDF "can save their place" in the PDF rather like putting a physical bookmark in a physical book - hence the name - and you can set the properties of a PDF bookmark to not only jump to a particular page but also to jump to a particular part of the page (using X Y co-ordinates) and to change the zoom factor. So if a page of the PDF contains a map, for example, it is possible to create a bookmark which, when tapped, will take you to a particular part of the map and zoom in at 200%.

By far the main use of PDF bookmarks nowadays, however, is as a convenient alternative table of contents to navigate the PDF. Rather than having to keep jumping  back to the table of contents at the start of a document, the user can use an equivalent series of bookmarks which appear in a separately scrollable pane on the left of the PDF viewer. The bookmarks can be structured in the same way as the table of contents with first level bookmarks and with sub-bookmarks. The person viewing the PDF can, of course, add their own bookmarks as they wish, choosing a zoom property which matches their individual requirements, but most bookmarks are created as part of the original PDF. It is important that these bookmarks do not have properties which in effect make assumptions about how the user will be viewing the PDF - some users will have large screens and some smaller. Indeed the same user may at different times use a large screen at their desk or a smaller tablet or laptop screen elsewhere. Someone using a "landscape" laptop screen which is somewhat less than the width of an A4 page will probably choose a zoom factor of "set visible width" so that, although they cannot see all of a page at once, that part of the page which they are displaying at any one time will not be too small. On the other hand someone using a larger "portrait" screen which has a display area which is as long as A4 but significantly wider than A4 size will probably choose a zoom factor of "fit page" so that they can see an entire page at approximately 100% zoom. If someone using the larger screen had selected "set visible width" that would have produced well over 100% zoom and unnecessarily resulted in the bottom of the A4 page being obscured. Conversely with the smaller screen "fit page" would have resulted in perhaps 50% zoom making the page harder to read. So the user needs to be able to choose the zoom factor they wish to use and have that choice honoured and to ensure that this is so the bookmarks created as part of the original PDF should have a zoom property of "inherit". This means that when the user taps the bookmark the zoom factor will remain what it was before - if they have chosen "fit page" and they tap a bookmark, the page they are taken to will inherit that zoom factor and also be displayed with a zoom factor of "fit page".  

If, as is usual, each entry in the table of contents at the start of the PDF is hyperlinked the links should have a zoom property of "inherit" for the same reason.

Page Sizes

If the contents of the PDF have not been produced from scratch but have appendices containing "source material" or if the PDF is actually a collection of disparate documents - for example an eBundle for legal proceedings - the different documents within it may have differing original page sizes. If, when viewing a PDF, you select a zoom option of, say, "fit visible width" and select the "single page" option you can go from page to page by tapping the "next page" and "previous page" buttons and each page, whether it is an A4 page or an A5 page or an A3 page, will be displayed fitting the width of the viewer window area. But if, instead of using the "single page" option, you choose "continuous" so that you can get from page to page by scrolling, as you may want to if you are using a touch screen, then having pages of different sizes in the PDF - A4 and A5 for example - can produce unwelcome results particularly for smaller screens. You do not want to scroll down from an  A4 page which just fitted the width of the view area and find that an A5 page following with small writing is unnecessarily miniaturised. Or if you are displaying an A5 page with small writing just fitting the width of the view area you do not want the following A4 page to be partly obscured - you probably want the A4 page to also be fitted within the width of the view area as well. So generally you will want all pages in the PDF which were originally A4 or smaller to be exactly A4 size in the PDF. 

With regard to documents which were originally of greater size that A4, that is A3 or A2 for example, opinion is divided as to the best way of arranging them in a PDF given that most people will using a size of screen which means that the view area in the PDF viewer is less than A3 size. Having all documents which were originally larger than A4 as exactly A4 in the PDF makes scrolling easier but may mean that fine detail cannot be immediately seen without zooming in. On the other hand it can be argued that having the whole page in view to begin with is a necessary first step before the user decides which part of the large page to zoom in on.   

Note: The PZ zoom factor and eBundles.  

In the early days of PDF usage some of the available software had its limitations. Adobe Acrobat, for example, used to have some unhelpful defaults for the zoom factor for newly created hyperlinks and bookmarks: the zoom factor when creating a new bookmark or hyperlink was "fit width" rather than than "inherit". It was possible to change the zoom factor of each bookmark and each hyperlink after it had been created but only by laboriously going through a series of panels for each bookmark or each hyperlink. For reasons such as this there was a potential problem of eBundles being created with settings which hampered their use by the court. Consequentlv the guidelines for eBundle issued by some courts at the time had stipulations which might seem, to the modern user of PDFs, obvious and unnecessary to state, and a modern reader of such guidelines might wonder whether they have correctly understood them or were missing something. 

A notable example of guidelines initially written some time ago which might seem to be stating what, to the modern user of PDFs, would be obvious, is specification of the PZ factor for a page. The PDF format standard allows a "preferred" zoom factor (PZ) to optionally be specified for a particular page. This is defined as 

"The page’s preferred zoom (magnification) factor: the factor by which it shall be scaled to achieve the natural display magnification."

It is easy to misread this. This optional field is not the zoom factor which users viewing that PDF page might prefer to use when viewing the PDF. Rather it is - speaking anthropomorphically - the page's own "natural" preferred zoom factor. Where the content of a PDF page was originally downscaled during capture the PZ attribute can optionally be used to record the magnification factor which would be necessary to exactly undo the downscaling. So an A4 page scanned into an A4 PDF page would have a PZ of 100% whereas an A3 page scanned in as an A4 PDF page would have a PZ of 140%. A PZ factor is unlikely to be appropriate for any page in an eBundle because if it is decided that A3 pages in an eBundle should be downscaled to A4 size to improve the scrolling experience then you do not want the PDF viewer software to try, prompted by a PZ factor, to undo this deliberate downscaling. If, on the contrary, it is decided that A3 pages should actually be A3 in the eBundle then there is no downscaling to be undone in any event. So there should be no PZ factor for any page in an eBundle - or the PZ factor should be 100% which is the same as if there were no PZ factor. As the PZ factor was originally defined for use in the specialised procedure of Web Capture it is unlikely that any modern software designed to be used to create an eBundle for legal proceedings would be designed to set a PZ factor, and nowadays few people, apart from IT specialists, will be aware that it is even possible for any kind of zoom factor to actually be specified for a page itself (as distinct from a bookmark or hyperlink) but in the Practice Direction of the UK Supreme Court - which, in 2009, was the first court in the UK to require the use of PDF eBundles, and to issue guidance about their format - it was apparently considered desirable at the time to expressly state that The default display view size of all pages must always be 100%. A modern creator of an eBundle, reading the Practice Direction and unaware that it is possible for a zoom factor to be specified for a page, might initially mistakenly think that bookmarks or hyperlinks were required to be set to 100%, rather than to "inherit", but that this is not what the Practice Direction intends is clear from the fact that attached to the Practice Direction is a sample eBundle PDF in which every bookmark and every hyperlink has a zoom factor of "Inherit". 

Disclaimer

This information page is designed to be used only by clients of John Antell who have entered into an agreement for the provision of legal services. The information in it is necessarily of a general nature and will not be applicable to every case: it is intended to be used only in conjunction with more specific advice to the individual client about the individual case. This information page should not be used by, or relied on, by anyone else.

The information on this page about specific computer techniques is provided for information purposes only. Every reasonable effort has been made to ensure that the information is accurate and up to date at the time it was written but no responsibility for its accuracy, or for any consequences of relying on it, is assumed by me. You should satisfy yourself, before using any of the techniques, software or services described, that the techniques are appropriate for your purposes and that the software or service is reliable.

This page was lasted updated in February 2024. Disclaimer