Converting scan into same resolution PDF

[macstatic]

I'd like to convert several scanned TIF image files into PDF (so I can combine them into a single PDF file), but somehow I'm not able to keep the original resolution (I've tried exporting from both Preview and Photoshop).
I can't remember the details from when I scanned them, but Preview's Tools-Show Inspector menu tells me they're 300 DPI with 3508x2480 pixels. The final edited files are slightly smaller in pixels as they're cropped, but still 300 DPI.

When I open one of the TIF images in Preview the "Scale" button shows 31% (which fills the screen), but when I open the same, previously exported PDF image it shows 100% (and also looks a little smaller on the screen). Any ideas what I'm doing wrong?

 

[usagora]

I'd like to convert several scanned TIF image files into PDF (so I can combine them into a single PDF file), but somehow I'm not able to keep the original resolution (I've tried exporting from both Preview and Photoshop).
I can't remember the details from when I scanned them, but Preview's Tools-Show Inspector menu tells me they're 300 DPI with 3508x2480 pixels. The final edited files are slightly smaller in pixels as they're cropped, but still 300 DPI.

When I open one of the TIF images in Preview the "Scale" button shows 31% (which fills the screen), but when I open the same, previously exported PDF image it shows 100% (and also looks a little smaller on the screen). Any ideas what I'm doing wrong?

I think it might just be the way Preview handles PDFs vs image files in terms of scaling. I just created a 3508x2480 300ppi image in GIMP and saved it in TIF format. In inches it measures 11.693 x 8.267. I then opened that TIF file in Preview and went to File > Export and saved it as a PDF. When I opened Inspector on the PDF, it says it measures 11.7 x 8.27 inches, so it seems to be the same resolution. When I zoom in on both the original TIF and the PDF side by side so they're the same on-screen size, I can't see any difference in the resolution:

 

[macstatic]

Interesting! So both the TIF and PDF files are in the same resolution but Preview reports their resolution incorrectly?

With this in mind I did a similar test by opening up a couple of those TIF files in Affinity Photo, then saving them as PDF files (I also have Photoshop CS4, but although it (just barely) works in OSX 10.13 High Sierra I just found out that the "File"-"File info" menu just gives me a blank window which can't be exited except by doing a force quit of the app)...

So I opened up (in Preview) some of the original TIF files as well as their PDF counterparts (exported from Affinity Photo) to compare. Here's what I found out:
The TIF file fills the screen at 31%, so I zoom in to "Actual size" (100%) and select a section of the image.
Next I open up the PDF counterpart (which fills the screen and already shows 100%, but nevertheless I zoom in further to get the approximately same size as the TIF file and scroll till I get to the same section of the image in order to compare with the TIF.
I end up at 400% with the PDF, which is the same size as the 100% with the TIF. The PDF is slightly darker and less detailed, but there's no pixelation and I suppose it's just due to some slight compression or loss in the conversion process. Past 400% I see pixelation starting to form (likewise with the TIF version).

So it appears you're right: although the PDF says it's at 100% in Preview it's actually not, and can be further zoomed in. There are options for defining 100% for images and PDFs in Preview, but I've tried both settings and they make no difference to this problem. Is this a reported bug?
I'm still using MacOS 10.13 (High Sierra), so obviously my version of Preview (10.0) won't be fixed, but perhaps newer versions will report the resolution correctly? Which OS version are you using, Usagora?