Using Hook with the Free Skim PDF Reader app: Deep Links!

Hook enables you to link PDF files to and from anything that is also linkable. Hook supports many PDF readers including the excellent, free PDF reader, Skim, available on

Hook treats PDFs similarly to any other files, except you can create “deep links” that point to specific locations in a document.

Click on the link and Hook will open the PDF and scroll to the specific page and location to which it points!

Copy Link, Copy Markdown Link and Hook to Copied Link

By default, Hook’s Copy Link and Copy Markdown Link in Skim copies links of the form hook://file// which point to the entire PDF file. You can paste those links anywhere.

In addition, you can create deep links to specific locations in a PDF (i.e., a specific page number and place on that page.)

Creating Deep Links (to specific locations in a PDF file)

You can paste the link in:

  • any document (for instance in Pages, Nisus Writer, or Microsoft Word), even in another PDF !
  • a “to-do” app (such as OmniFocus, Things or TaskPaper),
  • an email
  • in any editable field (even Items in DEVONthink )
  • and just about anywhere else

Share deep links to PDFs with others!

Before Hook, there was no easy way to refer to shared PDFs in Dropbox or iCloud. Hook solves this problem. Use the Copy Link (or Copy Markdown Link) command on a PDF or a selection of the PDF, then paste the link into a document or email. Send it to anyone with access to the PDF: If they have Hook (even if they use its free Lite mode), the deep links will open at the exact location in the PDF.

That way, instead of manually scrolling to the location in a (potentially very long) PDF, all they need to do is click on the link.

Hook links to specific locations in PDF files that are shared with others will normally work as long as the recipient has Hook and Skim installed and a copy of the file that has the same unique name on the recipient’s Mac (if two files have the same name, Hook will try to disambiguate them based on its meta-information [not its content]).

Try it, you and your recipients will be amazed at the time (and communication) savings!

Default app for opening PDFs

In order to use Hook’s deep links to PDFs you currently need to ensure that the default app you have configured in macOS Finder to open “.pdf” files supports deep PDF links. (As of 2021-01-07 this includes Skim and PDFpenPro. Additional apps may be added). See Choose an app to open a file on Mac – Apple Support.

Even today, however, Hook also supports deep PDF links with Curio using an alternative integration. In Curio, PDFs are not stand-alone files that you directly access with Finder. They are embedded in Curio documents.

Switching between PDF readers, and compatibility of deep PDF links

Hook’s deep PDF links are designed with the same standard-compliance principles and philosophy as Hook’s email links. (See Openness).

Where Hook’s email URLs (hook://email/<ID>) utilize RFC 5322 compliant email identifiers, Hook’s deep PDF URLs (hook://file//<...>p=<page-number>&s<starting-point>&e=<end-point>) use coordinate information specified in PDF specifications.

This means you can switch from one PDF app to another — as long as the other app supports automated deep linking. You can copy a deep PDF link in one app ( Skim) and open said deep links in another app (PDFpenPro).

Preventing deep links from being formed

If you’d rather get a link to the entire PDF, rather than a deep link, then prior to using the Copy Link or Copy Markdown Link command, simply clear the selection in Skim. You can do this by option-clicking in the margin of a PDF in Skim (or close and reopen the PDF file).

Hook to New > Skim

Hook does not currently support the creation of new PDFs. So there’s no Skim item in the Hook to New menu.

Searching for PDFs that you’ve bookmarked with Hook

To find PDF files you’ve linked with Hook, simply

  1. invoke Hook,
  2. enter search mode (with ⌘F or a click of Hook’s search toolbar-button),
  3. type ".pdf" and hit the return key (for a more precise search: "pdf" url:"hook://file")

Your PDF bookmarks will show up in the search results. This is a very handy trick, so it’s a good idea of getting into the habit of using Hook’s search feature.

Think of it this way: if something is important you’ve likely bookmarked it with Hook (e.g., by copying a link to it). So use Hook to search for relevant information.

PDF coordinates

When Hook creates a deep PDF link, the URL contains coordinates in the PDF. Page -level precision of Hook’s links is interchangeable between compliant apps, meaning you can create a link in Skim and consume it in (say) PDFpenPro or (in Hook 2.3) Adobe Acrobat.

However, the within-page coordinates of Hook’s deep PDF links in Skim depend partly on the type of Skim selection tool you use (Skim’s Tools menu).

  1. If Skim’s selection tool is set to ‘Select Tool’, then the deep URL will contain the page number, x, and y coordinates of the selection, like this: #p=4&x=60&y=213 .
  2. If Skim’s selection tool is set to ‘Text Tool’, then the deep URL will contain the page number, and search coordinates (start and end) of the selection, like this: #p=4&x=0&y=0&s=2114&e=2271 .

In either case, you get the page number, and some offset information. The page number is useful for any compliant PDF app. However, the search coordinates (returned by the Text Tool) can only be used by Skim.

That means that if you use the Text Tool to indicate deep links, Hook will only take you to the correct location on the target page if you open that URL in skim.

For completely interchangeable within-page precision of deep links from Skim, use the Select Tool.


This requires Hook 2.2 or later with integration scripts v. 158 or later.

Readdle’s PDF Expert

We’re sometimes asked about Readdle’s PDF Expert, so we mention here that as of this writing, PDF Expert for macOS is still not a linkable app as defined in What’s a Linkable App and Why Does Linkability Matter? – Hook.

See also Contacting Developers of Other Apps and Information for Developers – Hook.

See also…

Luc Beaudoin’s two Cognitive Productivity books explain in great detail how you can use the Skim PDF reader to boost your cognitive productivity.

Skim is also discussed and illustrated elsewhere on this website. To find out more: