Why does adding a text annotation in preview app increase pdf files size by over 10 from 38 kb to 486 kb?

Apple's native Preview app has tools for annotation, though some 3rd party PDF editors offer more options along those lines.

Info from Apple on Preview's annotation capabilities: Annotate a PDF in Preview on Mac - Apple Support

The file size jump when adding text annotations in macOS Preview is primarily caused by how the Apple Quartz PDF engine embeds full fonts and handles unoptimized incremental saves. 

Here is exactly why this happens in Preview, and why a third-party editor like PDF Marker avoids the problem:

Full Font Embedding vs. Subsetting

When you type text into a PDF, the editor must ensure that anyone opening the file later sees the exact same font.

• Preview’s approach: Apple’s engine often embeds the entire font file (or a very large, unoptimized data set) into the PDF structure the moment you use a text box. If you use standard macOS fonts like Helvetica or Lucida Grande, embedding all the metadata, character maps, and weights for that font instantly adds roughly 400 KB to 1 MB of overhead to the file.
• Third-party editors: Tools like PDF Marker usually employ "font subsetting". They only embed the exact characters you typed (e.g., if you type "Hello", they only embed data for those 5 letters), which adds virtually zero bytes to the file.

Less Efficient Encoding and Streams

Preview is built on Apple's universal Quartz graphics layer rather than a dedicated, highly optimized corporate PDF engine (like Adobe's or specialized third-party kits). When Preview modifies a PDF, it frequently rewrites the file's content streams using less compact compression, or appends a massive "appearance dictionary" to ensure the new annotation conforms to Apple's system-level rendering pipelines. 

Append-Only / Incremental Saving

When you hit save in Preview, it often doesn't compress or rebuild the document from scratch. Instead, it uses an "incremental save" that keeps the original 38 KB structure exactly as it was and appends the new data (the text box, font metadata, and rendering instructions) to the end of the file. This causes the file size to accumulate exponentially with every edit.

How to Prevent This in Preview

If you still want to use Preview but need to keep file sizes small, you can try these workarounds:

• Force a Rewrite (Save As): Instead of hitting Cmd + S, hold down the Option key, go to the top menu, and select File > Save As.... Saving it as a completely new file under a different name can force macOS to discard unneeded structural overhead.
• Use Quartz Filters: You can strip out heavy elements by going to File > Export, and selecting Reduce File Size from the Quartz Filter dropdown menu.
• Stick to the Top-Listed Fonts: Users have noted that using default system application fonts located at the very top of Preview's text font dropdown add significantly less bulk than selecting complex external fonts.

If you are dealing with a large volume of files or need strict file size limits for email or web uploads, sticking with your third-party PDF editor remains the most efficient solution

One can always change the PDF Reader. I took the same 119 Kb PDF and duplicated, and renamed it. The first of these, I added a multiline Text box annotation, and saved it. The second one, I added the same text annotation and saved. Same font, same font size. Here is the final result:

App Before After

Preview 119 Kb 1.8 Mb

Acrobat Reader 119 Kb 489 Kb

You can draw your own conclusions.

This is really underwhelming by Apple. Just reproduced a case where a file with 6kb went to 600 kb. That is insane. And it is *not* fonts being stored in the file. Will have to play with older macOS versions to check if they were as bad as Preview app under Golden Gate.

AutoZuLaut wrote:

Will have to play with older macOS versions to check if they were as bad as Preview app under Golden Gate.

I can save you the trouble there. If you're complaining about 486 KB and 600 KB, then it sounds like Golden Gate has made improvements in this area.

Apple's PDF generation is simply not very good. In past years, it's had serious bugs. I'm not joking when I say the past couple of years have been really good for Apple PDF.