Stop changing the shape of the text.
Keep the correct ratio.
You are not just scaling it down, you are changing the shape. Stop that!
Oh, and double posting is NOT allowed.
Sorry for posting it twice, but still not getting any solution.\
BTW I am not changing the shape of image and of course maintaing the correct ratio.
Those two things are not the same shape. If what I am about to say is too basic, please forgive me. It might help someone else later.
The ratio of your captured image (1434 x 958) is calculated as:
1434 / 958 = 1.496868475991649
The ratio of your export is calculated as:
640 / 480 = 1.333333333333333
Just get the representation of them into one document, I created a Photoshop file big enough to hold them both. Then I scaled it down and took a screen shot to fit this forum's restrictions on file size.
If you look at the first image, it is difficult to see that they are different shapes.
In order to make them easier to see, I scaled down the larger layer, using the Shift key to ensure that the scale is perfectly proportional.
Now you can easily tell that the ratio of 1434x958 is much wider than the ratio of 640 X 480.
That means that you are changing the shape of the image when you export.
If you are concerned about the long edge, you could divide 640 by 1.496868475991649 and export the image at 640X428 and get closer. It is not optimum, but should look better. Optimum would have been to size your PDF to where the longer edge was evenly divisible by 16. But, too late for that.
Another choice would be to edit the video in a 640X480 sequence, and scale the PDF video down to fit that sequence - leaving a letterbox look (black bars on top and bottom). That might help the image stay sharp. Then export to 640X480 from there.