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 bookmarks created as part of the original PDF do not in effect make assumptions about how the user will be viewing the PDF and force a particular zoom setting upon them, as different users will have different requirements. Some users will have large screens and some smaller. Indeed the same user may at different times use a large screen at their desk and a small tablet or laptop screen elsewhere and they are likely to use different zoom settings for each. Someone using a "landscape" laptop screen which has a width approximately the same as the width of an A4 portrait page and which is not a touchscreen will probably choose a zoom factor of "fit to width" so that when they go from page to page using the "previous page" and "next page" buttons, with "single page" set, the full width of each page is shown, whatever its exact size, so that they can at least easily read the full width of the page even though lengthwise not all the page will be visible at any one time.
However if they are using a somewhat larger landscape screen, again not a touchscreen, they will not want to use "fit to width" because that would expand a portrait page more than is necessary to read the text and, at the same time, reduce the amount of the page lengthwise visible at any one time. So they may, depending on page sizes, use a zoom factor of, say, 100% to ensure that they can still read the full width and have as much visible lengthwise as possible. If they have a very large desktop screen then they will probably find it convenient to choose "fit page" so that the whole of each page is visible at once, whatever its exact size and orientation, as they go from page to page using the "previous page" and "next page" buttons.
If, on the other hand, they have a touchscreen they will probably use "continuous" rather than "single page" so that they can move from page to page by touch and then the zoom factor setting will have different effects. With "continuous" in effect, selecting "fit to width" or "fit page" normally causes the zoom factor to change such that the largest page in the whole PDF (not the current page) will fit the width, or fit completely, as the case may be. Thus if there is one very large page in the PDF selecting "fit page" may cause the zoom factor to be reduced to, say, 10% so that most pages are then too small to read! To avoid this effect they will probably choose a zoom factor of 100%.
So the user needs to be able to choose the zoom factor they wish to use and have that choice honoured (rather than any particular zoom factor being forced on them when they tap a bookmark) and, to ensure that this is so, the bookmarks created as part of the original PDF should have a zoom property of "inherit". "Inherit" 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 they have chosen 100% and they tap a bookmark the zoom will remain 100% etc.
If, as is usual, each entry in the table of contents at the start of the PDF is hyperlinked the links should also have a zoom property of "inherit" for the same reason.
Can a page have a default zoom factor?
It is possible for bookmarks and hyperlinks to have a "default" zoom factor - "default" in the sense that it is the zoom factor to be initially applied when displaying the target page by tapping on that bookmark or hyperlink, but can of course then be changed. As discussed above for bookmarks and hyperlinks created as part of the original PDF it is desirable that there should be no default zoom factor (i.e. it should be "inherit" so that the current zoom factor, whatever it is, is used for the target page) but for bespoke bookmarks - additional bookmarks created by a particular user for their own use - a default zoom factor may be appropriate. A PDF page itself, however, cannot have a default zoom factor so that, for example, if you get to a page by scrolling, doing a Find, or by typing its page number into the PDF viewer, the zoom factor used to display it will remain at the current setting.
Guidance for PDF eBundles for use in the Supreme Court, however, specifies "the default view size of all pages should be set to 100%" which at first blush might seem to suggest that PDF pages can have a default zoom setting. The key to understanding this lies in the sample electronic bundle which accompanies the Supreme Court guidance. It is possible to specify an "initial page" for a PDF which is the page the viewer initially displays when the PDF is first opened - usually set to 1 - and if an initial page is specified an initial zoom setting can also be specified. If, in addition, every bookmark and every hyperlink has a zoom factor of "inherit" the initial magnification specified for the initial page will be carried forward (upon scrolling, tapping a bookmark, tapping a link, typing in a page number, or doing a Find) to every page subsequently displayed (unless and until the user choses to change the zoom at which point the new user-selected zoom would carry forward). This could be described as a default for "all pages". In the sample electronic bundle which accompanies the Supreme Court guidance the "initial page" is set to have a magnification of 100% and every bookmark and hyperlink has a zoom factor of "inherit" so this is obviously what is meant by the guidance that "the default view size of all pages should be set to 100%".
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 in the case of an eBundle for legal proceedings - the different documents within it may have originally had differing page sizes but nevertheless it is generally better for a consistent page size to be used throughout the final PDF. It is true that as long as you are using the "single page" option pages of different sizes may display satisfactorily with "fit width" or "fit page", and you can go from page to page by tapping the "next page" and "previous page" buttons. 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 A3 for example - can produce unwelcome results particularly if a smaller sized screen is being used. For example you do not want to scroll down from an A3 page which just fitted the width of the view area and find that an A4 page following is unnecessarily miniaturised. Or if you are displaying an A4 page just fitting the the view area you do not want the following A3 page to be partly obscured - you probably want the A3 page to also be fitted within the view area as well so that you can see the whole A3 page initially (you can then always zoom into particular sectors of the A3 page as an when required). So typically you will want all pages in the PDF to be a consistent size - typically A4 - even if originally larger. When an A4 PDF page which was originally A3 comes to be printed, it can, of course, be printed on A3 paper by specifying "fit to printer margins".
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