If you work much with PDF files, you may find that Preview isn't quite powerful enough for your needs. While Preview, especially in 10.6, includes many annotation features (circles, lines, comments, etc.), that's about the extent of its powers--you can't easily insert an image into an existing PDF, for example, nor can you modify the text within that PDF.
SmileOnMyMac's PDFpen not only marks up PDFs, but it makes short work of inserting images and even modifying, to some degree, the text within a PDF. It's also got one very unexpected feature, especially in a program with a reasonable $50 price tag.
Work with PDFs
The heart and soul of PDFpen is marking up and modifying PDFs, starting with a full set of PDF markup tools. Just as with Preview (the 10.6 version at least; the 10.5 version was more limited), PDFpen lets you easily add notes, text boxes, links to other pages in the PDF or to external URLs, and various shapes and lines to your PDF.
Going beyond Preview, PDFpen includes comment boxes (which can be turned on and off for printing, unlike text boxes) and additional shapes--Preview offers only rectangles, ovals, and lines; PDFpen adds polygons, rounded-corner boxes, and a free-form scribble tool.
Objects can be customized via the Inspector palette. Draw a square, for example, and you can set the fill color and opacity, line color and opacity, and line thickness and type. Depending on the type of object you're using, some features may not be customizable--I was unable to change the line type on a rounded corner box, for example.
Additionally, I found that changing the color of an inserted note was problematic--occasionally changing the note's color with the note open would also cause the text itself to change color. This issue has been reported to the company, and it will be fixed in a future minor update.
You can also, in theory, work with images in PDFs--moving, copying, resizing, and even deleting them. In my tests, the usability of the feature depended on the PDF being worked with.
In some documents, attempting to move an image would only move part of it; in others, I could move and resize at will. Sometimes, after moving an image, when I undid my moves, the image wouldn't move back to exactly where it started. Copying, though, always worked, which made it easy to extract images from PDFs.
You can edit text in PDFs using PDFpen, with certain limitations. The first limitation is that, depending on the document's font and layout, your replacement text may not be a perfect fit.
In my tests, sometimes I had a near-perfect result; other times, the line and/or character spacing was notably affected, or the font's appearance was subtlety different.
The second limitation is that you can't replace huge swaths of text and think that PDFpen will reflow your document; it won't.
For these reasons, the text replacement tool is best used on a few words (or even characters) that you need to replace, and for whatever reason, you only have access to the PDF itself, and not the source document.
I also experienced a PDFpen crash when trying to replace text in one extra-large (40MB) PDF; this was the only PDF I had that problem with, and the company is working on the problem.
Use the library
PDFpen includes a special Library palette, where you can store often-used shapes, scribbles, graphics, and text. The Library palette also includes a Proofreading Marks tab that contains a set of over 100 standard editing marks--a slash for delete, sideways "S" for transpose, etc. If you edit papers, these pre-made shapes can save you a ton of time. (Mouse over any of the editing marks and a tool tip will appear, defining each mark.)
Objects can be dragged from the library into your PDF, and you can modify their properties as you would any other object. I don't have a ton of things stored in the library, but it is a great place to keep the scanned version of my signature--I can now quickly "sign" any PDF I need to without hunting around for the source file on my hard drive.
Unfortunately, you can't resize the library palette (other than vertically), so larger objects will be small and hard to read; if the palette supported Quick Look-like previews, this wouldn't be a problem. Alas, it doesn't, so the only way to see what a small graphic may be is to insert it into your document.
You can insert any graphic into a PDF, and then change the graphic's size, crop, and location. You can also set its transparency, at least for images with a solid background color. An adjustment window lets you click any one color to make that color transparent; a slider then controls the tolerance of the transparency, so you can set the exact level of transparency you want. I used this feature to remove the white background from my signature TIFF file, and it worked well. As long as the image your adjusting has a solid background, you should be able to get very good results using this tool.
Finally, one of the features I really like is PDFpen's highlighter. Like Preview, you can highlight (place a translucent colored line across selected text) by first selecting a block of text then applying a highlight--though PDFpen offers five colors (one customizable) to Preview's single highlight color.
But support for multiple colors isn't the main reason I like PDFpen's highlighter. I like it because of its alternative work mode--just select a highlighter color, and then click-and-drag across your text to highlight as you go. This feels completely natural, and brings back memories of highlighting my textbooks in college.
I was also curious how the five colors would be supported in other PDF readers, and was quite happy to see that both Adobe Reader and Preview had no issues with the colors, including my custom color.
As seen in the image above, the colors translated well between PDF readers. Notice, too, the relatively narrow highlight lines used by Preview; I prefer the thicker lines in PDFpen and Adobe Reader. My only real complaint about PDFpen's highlighter is that, as seen in the image, it washes out the text behind the highlight; this problem doesn't occur in Adobe Reader or Preview.
There are a number of other PDFpen features that you won't find in Preview. The program will open Word files (both .doc and .docx), and you can choose whether or not to print notes and comments.
The unexpected feature
Surprisingly for its modest price point, PDFpen includes an optical character recognition (OCR) engine, which converts the text on scanned PDFs into actual selectable (and hence, copyable and pastable) characters. I tested the OCR capabilities by scanning a monthly bill, a hard-to-read receipt from a retailer, and a page of handwritten text.
With both the bill and the receipt, I was impressed with PDFpen's OCR capabilities--it read even tricky sections of the bill accurately, though it did have some trouble with some of the more oddly-formatted sections of the page. The receipt had a couple of wrinkles in it, which I did my best to straighten out prior to scanning. Still, there were some dark lines in the scan, and the OCR engine had some trouble with these sections--as you might expect. Other than those sections, though, the OCR did a great job turning both the receipt and the bill into selectable text.
My handwriting, on the other hand, was completely beyond the power of the OCR engine. This shouldn't be surprising, as OCR software isn't typically capable of recognizing handwriting. (As a control, I fed the same scanned document to the WeOCR Server, which uses the open source Tesseract OCR engine. It was also unable to translate my handwriting.)
Many scanners today include OCR in their driver software. If yours doesn't, or if you're looking for a way to convert already-scanned documents to usable text without rescanning, PDFpen's OCR software may be a viable alternative to costly standalone OCR packages.
Macworld's buying advice
If you find yourself wishing for more PDF editing features than you get in Preview, or if you've got a need for OCR conversion of scanned documents, PDFpen is an excellent tool for the job. Its abilities compare well with Acrobat Pro, and yet PDFpen comes in at roughly a tenth of the cost. There are a couple of rough edges, but nothing that detract too much from the program's PDF editing skills.
[Rob Griffiths is a Macworld senior editor.]