Corrections in red.  Stylistic suggestions and comments in blue.

The Specifications of Paper.   

Is the text is written correctly? Is it clear what I'm talking about?

There are a number of technical characteristics of the [1] paper. They depend on the composition of the paper [2] and its manufacturing process. They also determine the properties of the future product. Here are some the specifications of paper.

Density
The density determines the weight in grams of a sheet size of one square meter.  From The density of the paper determines the print quality, and the ease of passage of the sheet through the printing device (copier, printer). Paper that has too high a density may become jammed in the apparatus, and if the paper has too low a density, the printer will grab several sheets at a time. For example, newsprint has a weight of from 45 to 60 g/m2; office paper has a weight of 80 g/m2; for leaflets and booklets use paper with a weight of 100 to 170 g/m2; and paper used for making cards, [3] 200-250 g/m2.
Weight is measured in grams per square meter.   [I haven't studied science in decades, but I think you are referring to weight.  As you said in the first sentence of this paragraph, "density determines <em>the weight in grams of a sheet size of one square meter.</em>]

Whiteness 
This characteristic concerns the ability of paper to reflect light absently [4 ?] and evenly in all directions. Than white above, [5 ?] the image is sharper and provides more contrast. The whiteness of paper is expressed by the reflection coefficient, i.e. the ratio of the intensity of the reflected light to the incidence. The coefficient ranges from 60% to 98%.
The whiteness of paper measured is measured in percentages.  [6?]

Transparency and Opacity 
These concepts have to do with the transmission of light through the paper without scattering. Transparency in most cases is due to the the lack of density of the paper, which results in a visible imprint on the reverse side of the sheet. [7]  To much transparency makes it impossible to do two-sided printing on this paper.  An opacity of 98% is considered the minimum for office paper.
Transparency and opacity are measured in percentages. [8]

Thickness
This indicator is important for the unobstructed passage of the paper in the printing device, and in order to know how durable the printed documents will be.
Thickness is measured in microns.

[1] If you were speaking about specific paper already mentioned, you would say "the paper."  But it appears that you are speaking about paper in general.

[2]  Here, the idea is that each specific type of paper has its own specifications.  The sentence is equivalent to saying, "They depend on the specific type of paper we are discussing."  That is why the article "the" should be used.

[3]  It is not clear what you have in mind by "cards."  There are different types of "cards."  For example, there are playing cards (игральные карты), business cards, and greeting cards (birthday cards, Christmas cards, etc.)  Perhaps it would be good to specify what type of "card" you mean.

[4]   I don't think the word "absently" is the correct word here, and I'm not sure what you mean.  Here, and for numbers [5], [6], [7], and [8], please paraphrase in English, or write the sentences in Russian, and I'll try again to help.

[5]  "Than white above" is not understandable.  

[6], [7], and [8]   These sentence is not understanable. I "guessed" at what you I think you wanted to say, but I'm not sure if I guessed correctly.

The Specifications of Paper.
Is the text is written correctly? Is it clear what I'm talking about?

There are a number of technical characteristics of the paper. They depend on the composition of the paper and its manufacturing process. They also determine the properties of the future product. Here are some the specifications of paper.

Density
The density determines the weight in grams of a sheet size of one square meter. From The density of the paper determines the print quality, and the ease of passage of the sheet through the printing device (copier, printer). Paper that has too high a density may become jammed in the apparatus, and if the paper has too low a density, the printer will grab several sheets at a time. For example, newsprint has a weight of from 45 to 60 g/m2; office paper has a weight of 80 g/m2; for leaflets and booklets use paper with a weight of 100 to 170 g/m2; and paper used for making cards, 200-250 g/m2.
Weight is measured in grams per square meter.

Whiteness
This characteristic concerns the ability of paper to reflect light absently and evenly in all directions. Than white above, the image is sharper and provides more contrast. The whiteness of paper is expressed by the reflection coefficient, i.e. the ratio of the intensity of the reflected light to the incidence. The coefficient ranges from 60% to 98%.
The whiteness of paper measured is measured in percentages.

Transparency and Opacity
These concepts have to do with the transmission of light through the paper without scattering. Transparency in most cases is due to the the lack of density of the paper, which results in a visible imprint on the reverse side of the sheet.  To much transparency makes it impossible to do two-sided printing on this paper. An opacity of 98% is considered the minimum for office paper.
Transparency and opacity are measured in percentages. 

Thickness
This indicator is important for the unobstructed passage of the paper in the printing device, and in order to know how durable the printed documents will be.
Thickness is measured in microns.