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.
Can a page have a default zoom factor?
Sometimes people talk about PDF pages having a default zoom setting for viewing. A PDF page as such doesn't have a default zoom setting. When people talk about all pages having a default zoom setting they could mean all bookmarks and hyperlinks in a PDF having a particular magnification (such as 100%) set so that whenever a bookmark of hyperlink is tapped the target page is then displayed with that magnification. However such an arrangement would be quite inconvenient for the user, unless that happened to be their preferred zoom, because it would mean that every time they tapped on a bookmark or hyperlink they would then have to change the zoom back to their preferred setting. So this is unlikely to be what is meant when people talk about all pages having a default zoom setting.
It is possible to set the properties of a PDF to specify the initial page to be displayed when the PDF is opened (usually the first page) and the zoom factor when that initial page is displayed. This could be described as a "default", and if all bookmarks and hyperlinks are set to a zoom of "Inherit" then this initial magnification of the initial page would then be carried forward (upon scrolling, tapping a bookmark or tapping a link) to every page (unless and until the user chose to change the zoom at which point the new user-selected zoom would carry forward). So it might perhaps be said that in this case the default "for all pages" was the zoom factor specified for the initial page. This seems to be what is meant by the Supreme Court's electronic papers guidance when it states that "The default view size of all pages should be set to 100%" - the accompanying electronic sample bundle has an initial page magnification of 100% with every bookmark and link set to "Inherit".
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